Sorry to hop in on this one but uhm, will this still work if I had accidentally entered nothing in when asked to submit the key? Seems I was a bit too eager and assumed a simple copy/paste would've done the trick - but I didn't see anything entered in and clicked the submit button anyway. ^_^;
Anyway, just tried that set of terminal commands, but no luck. Failing the ability to manually find and edit the file, would you reckon deleting the local content (on Steam) and re-installing the game will reset the ability to enter said serial key?