The pack a punch works slightly different in this map, as do the power ups.
It's worth noting that the progression is similar to the original, but there are differences.

Important note:
If you are alone, or have a full team but are having a hard time, change the difficulty to easy, set ammo to unlimited and turn up the damage multipliers as high as you can. This will at least give you a chance to figure out how to complete the map.

One other consideration to make, especially if you are playing on anything other than easy, is to keep one zombie alive at the end of each round so that your team can catch up on reloads, mystery boxes and finding and returning the fuses.

Phase 1 - Get points and starting weapons
You will spawn upstairs in the central hut as normal. If you're playing on a private session, as soon as the game starts, the host will have the chance to change the difficulty.

You should start the game by completing a few rounds upstairs to gain points. Headshots will give you more, and using the knife seems to get the most.

Before you get to the round with dogs, you'll want to open the first barrier leading to downstairs.

This will give you access to some more powerful weapons and the mystery box.

Stay in this area and keep buying weapons from the mystery box until it disappears. This is very important, as you wont be able to get the pack a punch weapons without exhausting the mystery box. It will also give your team a chance to get some good weapons.

Phase 2 - Hunt for the fuses.

Before opening one of the four barriers to the rest of the map, take a look around the corner from where the first mystery box is. You will see a panel on the opposite side of the wall with the familiar pack a punch logo.

This panel will have 4 sockets and some text to indicate which location the socket corresponds to. You will have to visit these locations in order to find the fuses to unlock pack a punch.

There is no specific order to which one you should complete first, as it is up to RNG where the mystery box will appear next. But you should coordinate with your team and go to each location together so whilst someone is completing the hunt for the fuses, the others can defend them.

In addition to this, each of the power ups will randomise on opening each location. To use power ups in this map, besides having enough points to buy one, you need to pop the cap off with a free hand using the trigger then manually drink it. I highly recommend getting each of these, especially the jugger-nog.

Comms room :

In one of the rooms you will find an old rotary telephone with the handset hanging by its cord. Interact with the handset repeatedly until you hear a voice coming from the phone then a song will play. Spam the mystery box until it disappears, keeping any weapons you like. Then check a shelf with a lantern on it with two barrels to the right for a fuse. It will look like a blue light bulb.

I have also seen it appear on the console on the porch leaving the comms room.

Grab it, pin it to your chest and run back to the hut to plug it in.

Doctors quarters :

As you run in, check the bookshelf on the left. Interact with it and you should get an audio response. Spam the mystery box until it disappears and you should find the fuse has appeared on another book shelf. Return it to the main hut.

Storage :

Make your way to the storage building, spam the mystery box then scour the building for the fuse. Return it to the main hut.

Fishing hut :

Enter, spam the mystery box and look in the back of the hut where there is a fishing rod looking out over the water. For me, the fuse has shown up in two locations, one could be on the left at the base of the fishing rod, I have also seen it on a crate inside next to a large cone shaped net. Grab it and head back.

I have had varying experiences with this, but it's worth noting that sometimes when I've reached the final location, which ever that was, I didn't need to spam the mystery box to unlock the fuse. If this happens to be the Storage or Fishing hut for you, just head straight to the fuse as it may already be there, thus saving you thousands of points on the mystery box.

After plugging in the final fuse, you should get an audio cue.

Congratulations, you have now unlocked pack a punch weapons.

Phase 3 - Survive!

Unlike other maps with a dedicated machine for upgrading your weapons, you can only get random ones from the mystery box. But it still beats not having one and makes the late game more interesting, especially if you have limited ammo on. You will find yourself trying new weapons rather than slogging out wave after wave listening to the same gun fire audio.

Final word :

This map allows the use of the infection mode game play. When a player fully dies and can no longer be revived, they rejoin the game as a playable enemy. This will remain until the rest of the survivors die, and the match is over. Leaving and rejoining will still place you on the zombies team. Only select this if you are okay with being a zombie.

There is also a couple of places in the map you can escape to for a break, be it that you need to reload your MG 42, or you just need to adjust your headset, get a drink, whatever.

Note: these may get patched by the creators of the map.

The first is in the main hut where you spawn upstairs. Face the stairs as if you're going down, keep to the left and you should be able to walk on the thin ledge on the upper floor inside the stairwell. Move to the far back and you should be safe.

The other safe location is on the way the Storage hut. This path has 3 lanes you can take, 2 in the water, and one that has a hut then ends with water. Go to the building that has the fuse, then turn around to face back at the main hut.

Go to the middle lane and hug the right hand side where the wooden ledge is. Continue walking along this until you end up on the raised section between the lanes. Immediately turn to face the way you came and gun down any zombies that followed you then you should be safe indefinitely.

Whilst you are there, zombies will not be able to find a path to you, and they will despawn.

To leave this safety zone, just walk to the edge of the raised area near the central hut and you will be able to hop off the edge.