To be honest I'm not surprised considering your using a mac. I own one too and it just won't game! It could just about manage worms 3d... just about! If you bought it and didn't download it then there shouldn't be anything wrong on the game's side.
Maybe give us the specs of your mac so that we can rule out hardware limitations. To be honest its probably the graphics card if its anything like mine. If your mac is before 2007/ 2008 then you might have to accept that your graphics card won't do it!