OK, I just bought the OSX version through Steam and I cannot figure out how to get started! This is not much like Portal :-) I'm faced with a large white cube floating in space. WASD make the cube rotate on 2 axes, mouse drags can alter its orientation somewhat. Each face of the cube represents a level, I get that; most are locked, only World 1 Level 1 is unlocked, I get that; but how do I start playing??? I have Level 1 facing me, and I've tried every key I can think of but the game never starts, I'm just staring at the big white cube. Where's the Start button???