Have to agree with skategoat there - Thule boxes are pretty much idiot proof.
For the one that I have you only need to unlock one lock to open it (the one master lock locks the box in three places). And you can't close it without locking it. And you can open it from both sides (master lock on both sides). That really helps when you mount/unmount the box.
One more advise - buy everything for your roof rack system from one company. That way you can get matching locks - I only need one key for all the components that I got for my Thule system (cross bars, bike rack, box and snowboard racks).
Hagen.