I enjoy this game for being lite survival, but for the life of me i can't find the last supply box to build the habitat in the amount given.  I have found the airlock, oxygen plant, construction table and bio fuel generator... but i can't find the cube you go into after passing the airlock regardless how many times I try.  Is this a bug? or do i have to craft it, and if so how? if not where would it spawn?

You start off with two crates located near the lander. By oxygen plant I am guessing the square with a fan in it. That's your starting room. Place your airlock to one side and you can go in. You need to make more, which unlocks after you research construction.

NobodyZero is correct, you'd connect the Airlock to the Air Cleaner / Habitat module (the one with the fan on it).  

Example of the usual early base layout... (ignore the old style wood floor)

Ooooh that makes sense... ok thanks for the help =)