Just to add clarification:
A .acsm file is a "Adobe Content Server Message File." It can be opened by Adobe Digital Editions.
There is a free Digital Editions 1.7.2 installer that can be downloaded from the Adobe website.
http://www.adobe.com/products/digitaleditions/?PID=3081440
NOTE: Even if you have a different eReader software other than Digital Editions, such as Kobo Desktop, you may still receive an ACSM file when downloading an eBook. However, you still must install Adobe Digital Editions to download the eBook and then it can be read and transferred to your eReader device.
Hope this helps.