The free version of Adobe Reader (i.e. the one that's freely available for download from Adobe) doesn't allow 'data import' - which is what I think you're doing with your XML file - unless the PDF has been magically 'unlocked' via LiveCycle Forms ES or Reader Extensions ES.
But, if the version of Reader you're using is the one that comes with Acrobat Professional (which also includes LiveCycle Designer), then the above doesn't apply, and as far as I know 'data import' should work.
I should point out that I'm not an Adobe expert, I'm just sharing knowledge I've struggled to obtain via the same route as you - i.e. having to rely on message boards and blogs.
If only there was an equivalent of MSDN for Adobe....
