You need to have Adobe Digital Editions installed. http://www.adobe.com/uk/products/digital-editions/download.html
That will open the .acsm files and download appropiate DRM .epub files (or .pdf files if you are unlucky).
ADE should associate .acsm files with itself. If not, do the association yourselft with windows 'open with'.
You can open .acsm files in ADE using drag-drop from explorer onto ADE, but for some silly reason you can't open them with file open (ctrl-O).