ADE is silly with opening .acsm files.
You can open them by drag-dropping from explorer or from command line (eg by associating .acsm with ADE),
but not using the standard open command.
Associating .acsm with ADE is usually the easiest once set up, but can be problems, see ~~~ below
The .epub book is what you want, the .acsm is a step on the way towards getting the .epub.
The .acsm is a sort of token file allowing download, and the .epub is the real book (encrypted for DRM books).
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Sometimes the registration of ADE to Windows as the default hander for .acsm and .epub files does not work;
that is what causes the double-click in Explorer to fail;
especially in Windows 8/8.1, and maybe 7;
this appears to be a bug in Windows (for once not a bug in ADE).
Two possible ways round it.
First, try to force it in Explorer
Right-click on a .epub file (and repeat entire sequence for .acsm if necessary)
Select 'open with' if on the list, 'open' if not.
Select 'choose default program'
Select ADE if it is on the list,
If not select 'more options' and navigate to the ADE executable (may be at C:\Program Files\Adobe\Adobe Digital Editions\digitaleditions.exe)
Now .epub/.acsm files should be associated with ADE, and double-clicking should work.
Sometimes, Windows allows you to go through all that, but then refuses to make the association you have asked for.
In that case, the only way I know is to use a third partly file association program.
http://www.softpedia.com/get/System/System-Miscellaneous/Types.shtml works for me.