I installed Digital Editions 2 on a Toshiba Laptop running Windows 7.
The installation program did not associate acsm files with Digital Editions.
Digital Editions is not on the list of program files that can be associated with file extensions. Windows will not allow me to manually associate with acsm files.
Windows association does behave oddly sometimes; and more stroppy about doing what you want with more recent versions of Windows than with older ones.
It might be worth trying to create a shortcut to Digital Editions and see if associating it with that works.
ADE will not open .acsm files in the usual way with ctrl-O.
You can open them if you drag/drop them on ADE.
Obviously you want to associate properly, but may be helpful as a standby.
I have overcome the problem by uninstalling ADE 2 and reinstalling ADE 1.7 from the address given in the thread below. I can now download books and read them using 1.7. Will retry ADE 2 when it becomes 2.1 and bugs are fixed.