Chonged,

The one @ Overgrow is kept outside the camber... and then you have your fan (fart fan, Dayton, etc.) that's inside the chamber... The fan pushes the air from inside to the outside...

I'm positive you can also do it the other way around... putting the carbon scrubber inside... but I would think it involves more work... it's easier to put it outside.

There's another FAQ @ Overgrow (I'll search for the link if you want) that has a very cool carbon scrubber made out of panels...