Aaah, I was ready to just close it and murmur that they didn't even test if the thing can be started on Ubuntu. That "start" totally looks like a button.
And after reading your comment I tried WASD first, which doesn't work.
Is it trying to be cute through subversion? Like that one puzzle in Braid that nobody could figure out.
If not, then it's just bad design; press "Start" is as old as game themselves, no design pattern more deeply ingrained in players minds.