On a Windows PC as soon as you right click on an acsm file and chose open with ADE, ADE will start and the book starts downloading.
You mention Borders in your opening post. Have you migrated your library to Kobo? If not here is
link with instructions on how to do that. In any event, you should try to redownload the acsm files and try again to open them with ADE. If that does not work you could try to uninstall and then reinstall ADE.
What version of Mac OS X are you using?