What pack did you buy? You should be able to select the armor by clicking at the marine in the customize menu until it displays the skin you want to select. Is the marine skin available in your steam inventory.

Recently got the Tundra and B.M.A.C. ELITE SUPPORTER PACK.

All of the skins work for the tundra except any related to the marine Armour.
I've check my steam inventory. They are present. When i click on the change the Armour upon reaching the one i want i switch out to a different tab and the Armour switches back to default.

I've also noticed which was weird all of the skin options bring up a buy option for the ones i don't currently own i get no such option with the marine armour.

Sounds like something may be off with your game client.

Run the x64/RepairGame.exe in the game's install directory -> Wait till it's done -> Restart the game and see if you still can't select the skin.

Also please check the game client's log.txt for any kind of error output.

Tired the first thing didn't work. I've also reinstalled the whole game before to try and fix it.

This is what i found in the log.
Client : 3.439644 : GetGeoCoords Response:
{"code":200,"_lat":54,"_long":-1,"td_enabled":true}
[ 13.309] MainThread : Error: lua/menu2/PlayerScreen/Customize/CustomizeScene.lua:1335: attempt to perform arithmetic on local 'patchIndex' (a nil value)
[ 13.742] MainThread : Error: lua/menu2/PlayerScreen/Customize/CustomizeScene.lua:1335: attempt to perform arithmetic on local 'patchIndex' (a nil value)
[ 17.241] MainThread : Error: lua/menu2/PlayerScreen/Customize/CustomizeScene.lua:1335: attempt to perform arithmetic on local 'patchIndex' (a nil value)
[ 17.391] MainThread : Error: lua/menu2/PlayerScreen/Customize/CustomizeScene.lua:1335: attempt to perform arithmetic on local 'patchIndex' (a nil value)
[ 17.558] MainThread : Error: lua/menu2/PlayerScreen/Customize/CustomizeScene.lua:1335: attempt to perform arithmetic on local 'patchIndex' (a nil value)
[ 17.708] MainThread : Error: lua/menu2/PlayerScreen/Customize/CustomizeScene.lua:1335: attempt to perform arithmetic on local 'patchIndex' (a nil value)
[ 18.042] MainThread : Error: lua/menu2/PlayerScreen/Customize/CustomizeScene.lua:1335: attempt to perform arithmetic on local 'patchIndex' (a nil value)
[ 18.224] MainThread : Error: lua/menu2/PlayerScreen/Customize/CustomizeScene.lua:1335: attempt to perform arithmetic on local 'patchIndex' (a nil value)
[ 18.407] MainThread : Error: lua/menu2/PlayerScreen/Customize/CustomizeScene.lua:1335: attempt to perform arithmetic on local 'patchIndex' (a nil value)
[ 18.891] MainThread : Error: lua/menu2/PlayerScreen/Customize/CustomizeScene.lua:1335: attempt to perform arithmetic on local 'patchIndex' (a nil value)
[ 19.091] MainThread : Error: lua/menu2/PlayerScreen/Customize/CustomizeScene.lua:1335: attempt to perform arithmetic on local 'patchIndex' (a nil value)
[ 19.258] MainThread : Error: lua/menu2/PlayerScreen/Customize/CustomizeScene.lua:1335: attempt to perform arithmetic on local 'patchIndex' (a nil value)
[ 19.407] MainThread : Error: lua/menu2/PlayerScreen/Customize/CustomizeScene.lua:1335: attempt to perform arithmetic on local 'patchIndex' (a nil value)
[ 19.824] MainThread : Error: lua/menu2/PlayerScreen/Customize/CustomizeScene.lua:1335: attempt to perform arithmetic on local 'patchIndex' (a nil value)
[ 20.374] MainThread : Error: lua/menu2/PlayerScreen/Customize/CustomizeScene.lua:1335: attempt to perform arithmetic on local 'patchIndex' (a nil value)
[ 20.824] MainThread : Error: lua/menu2/PlayerScreen/Customize/CustomizeScene.lua:1335: attempt to perform arithmetic on local 'patchIndex' (a nil value)
[ 25.640] MainThread : Error: 1 memory leaks in 'ClientWorldMisc' (24 bytes)
[ 25.640] MainThread : Error: 1 memory leaks in 'ClientWorld' (24 bytes)
[ 25.641] MainThread : Error: Attempted to load remote options from a file that does not exist.
Worker 25: exit

There seems to be an issue with the shoulder patch selection causing your marine skin selection to not be saved/applied. Which shoulder patch do you have selected?

Could you maybe provide your %APPDATA%/Natural Selection 2/options.xml ? Or at least what the X value in the options file for <shoulderPad>X</shoulderPad> is. Change X to 0 and see if that helps.

Will forward your issue report to a dev who worked on the "new" main menu.

So i opened the XML and change it from 2 to 0. However when i launch the game it switches back to 2 reappearing when i change skin but the file had already been change back to 2 before i enter the skin menu.