can't seem to add com addin in

I'm trying to add/register a COM Addin to interconnect with Raiser's Edge.
The only problem is when I go to do it, the File dialog that comes-up won't
let me select a file - any file. I click on any dll and the file open box
remains empty. No security error, it just doesn't select a file. I'm the
Admin on both machines I've tried it on - one with Outlook 2002, one with
2003.

Why can't I add a COM AddIn?

Thanks.