What exactly happens when you try to launch it? Do you get any error message? If so, please specify the error. An error screenshot will help.
Did you actually install Digital Editions after downloading it?
[Moved the discussion to Digital Editions Forum]
What operating system.
If XP, quite a few people have problems. One resolution is to install the older 1.7.2 version instead (which also avoids various other bugs).
Version 1.7.2 is a little difficult to find,
I found it for PC at http://www.tucows.com/preview/853736/Adobe-Digital-Edition-For-EBooks
I'm not sure where there is a Mac version.