Go to http://forums.adobe.com/community/premiere and enter
in the Ask a Question box
You may now read all the previous discussions of your question... be sure to click the See More Results at the bottom of the initial, short list
If you had a boxed version of Adobe, you would need to contact Adobe Support about possibly switching the licensing. For electronic download purchases, especially CS6 upgrades which are ALL electronic, you should already have licenses for Mac and PC in your Adobe Online account, so check there first and see what is available before contacting support.
Regarding Mac, you must have your reasons, but from all the posts I see on the subject, PCs seem to provide more bang for the buck with Premiere and certainly more options for graphics cards for GPU acceleration with the Mercury Engine. Something to consider.
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