They've seriously goofed on most of their games. If you just change the user agent in Safari to "Windows MSIE 6.0", all the games play just fine.
Interestingly, you can also play the games if you just ignore the "The Mac platform doesn't support Livescript" window. Keep it open, and just move it to the side. As long as you keep it open, you can play 'til you turn blue in your face wihout a hitch.
I hate this kind of moronic programming. Browser testing is almost always implemented the wrong way: looking for a specific set of browsers, instead of just testing if the browser actually supports the things you need it to support. Who knows what browsers will be available tomorrow, and what feature set they will have?
The programmers responsible for the games on Yahoo seems to be mostly pinheads, doing it the wrong way.