the real issue is less to do with Windows/Mac OS and more to do with what browser you're testing the site on...
I'll assume for the purposes of this response that on Windows you're running Windows Internet Explorer, and that on Mac OS you're running Safari...?
According to Apple's web site, Safari should support iframes:
http://developer.apple.com/internet/safari/faq.html#anchor24
(I'm not near a Mac at the moment so I can't physically check it myself...)
So if you could provide the code which is not working, plus confirm which browser and which version of that browser you're using, we'll take it from there
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