Hi,
I think that XFA forms stored on Acrobat.com cannot be opened directly within Acrobat.com irrespective of the operating system. Here is Windows 7/IE8:
We have developed lots of forms in LC Designer ES and ES2 that open in Acrobat and Reader on a Mac (OS X).
When opening XFA Forms, are they opening in Preview - if so that may be the problem.
Good luck,
Niall