How do I add System Font in Acrobat 6.0?

When I open an Acrobat 5 file in Acrobat 6 Pro when trying to do Text TouchUP I get the following error:

All or part of the selection has no available system font. You cannot add or delete text using the currently selected font.

I have the full install Acrobat 6 Pro version but can’t do Text TouchUp on an Acrobat 5 file.

Any idea why?