For your free ebooks, try the free Calibre. Much better than ADE.
For ADE with DRM ebooks, try ...
Sometimes ADE gets its registration/activation confused and in a semi-authorized state.
Uninstalling and reinstalling does not help.
Unfortunately, it often then gives misleading error messages about what is wrong. (That means I'm not at all sure it will help, but ...)
A common incorrect message informs you that the ID is already in use on another computer and cannot be reused.
This can often be resolved by completely removing any authorization using ctrl-shift-D to the Library screen on ADE (cmd-shift-D if on Mac).
Restart ADE, and then reauthorize with your (old) Adobe ID.
In extreme cases on the mac, the following extra step has helped some people. Navigate to /Users//Library/Application Support/Adobe/Digital Editions and drag the activation.dat file to the trash. If you are using 10.7, see Access hidden user library files | Mac OS 10.7 Lion. http://forums.adobe.com/thread/1265248?tstart=0
For free ebooks I use iBooks, Nook, or Kindle. I only mentioned free ebooks here to show that my installed ADE works with them, but not on DRM material.
Also, as in original post, I'm using 10.6.8, not Lion.
I'm afraid trashing the activation.dat file didn't work - thanks for the suggestion.
Anyone else have any? Update is that ADE on my husband's Windows XP system accepted my Adobe ID and opened the file just fine. So it's definitely my system that's the culprit.
Another possibility is a bug in ADE2.0. I haven't seen this particular one, but replacing it seems to solve a wide range of issues.
There are lots of bugs in ADE2.0. Try replacing ADE2.0 with the older but more reliable v1.7.2.
(You can have them both installed at once if you like.)
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
The forum software is sometimes corrupting the link above. There shouldn't be a blank in 'editio ns.html'. The following redirects to the same page: http://tinyurl.com/diged172
Thank you, sjpt, but again, ADE 2.0 is working fine on my husband's system. The problem is on my system, not with the application.