layers and column issues on a Mac

Hi, I'm writing to see if any of the experts here can take a glance at my page prototype and help me understand why the columns are messed up when viewed on a Mac. The page is:

http://www.katemoennig.net/fbwd/main.html

The problem is that the center/right column content gets shifted over to the left, and this is only an issue with IE/Mac and the "normal" section of Microsoft Frontpage (PC). Under "preview," the problem gets fixed, but I don't know what the underlying issue is -- maybe layers?

thank you so much!!

Nadia