Are your scrollbars not showing up at all? I am doing something funky with the scrollbars in this, but if it's not supported you should just see regular scrollbars. What browser are you using?
On OS X, the default behavior hides scrollbars in several different circumstances. I can't stand it and I normally have them shown all the time.
The issue for me was that I wasn't expecting to scroll in a game. Nothing against scrolling in games, but it's not the paradigm I'm used to.
Maybe move the buttons "above the fold"? It kind of messes up your current layout, but I wouldn't want other people to miss playing your game because – like me almost – they didn't see the start button. Anyway, just a suggestion. Cheers.