the mission folder needs to be in the right place. The following is the layout on my machine, unless your name is Jim you'll probably have to change some bits!

Multiplayer missions that you play are kept in:
C:\Users\Jim\AppData\Local\Arma 3\MPMissionsCache

Multiplayer missions that you publish are kept in:
C:\Program Files (x86)\Steam\steamapps\common\Arma 3\MPMissions

Multiplayer missions that you're editing in Eden Editor are kept in:
C:\Users\Jim\Documents\Arma 3 - Other Profiles\Jim\mpmissions

Once you've 'played' the mission by hosting a multiplayer server, or digging in the steam downlads bit the pbo should be in MPMissionsCache.

Then copy the file to somewhere convinent and open the pbo in pbomanager. Highlight the files and right click > copy. (ctrl+c won't work for some reason)

Next go to
C:\Users\Jim\Documents\Arma 3 - Other Profiles\Jim\mpmissions

and make a folder called ION_2035.VR (be sure to include .VR .Altis or .Stratis on all your mission folders

paste in the files from pbo manager.

Open Eden Editor and open a multiplayer mission - you should see ION_2035.VR right there.

I'm sure this is as clear as mud - so I check out Tym Cooper's video

YouTube™ Video: Arma 3 - PBO Manager - Open Multiplayer Missions - Editor Multiplayer Missions
Views: 70,739
PBO manager: http://www.armaholic.com/page.php?id=16369
which is how i learnt all this.

Top tip - pin all these folders into the side pannel of file explorer/manager or make desktop shortcuts because they're a pain to find every time you need them.