If you open the Adobe Download Assistant has the download been completed? Often we see this issue occur when either the Applications folder has been chosen or when somone attempts to mount the DMG file before the download has been completed.
If the download is completed in the Adobe Download Assistant you can temporarily turn off the feature to verify the checksum. The following steps will allow you to temporarily disable this feature:
- Open the Disk Utility located in Applications/Utilities
- Go under the Disk Utility menu and select preferences
- Deselect the option to verify checksums
- Try mounting your DMG file
I would recommend turning this feature back on though after the installation is complete. Also if you notice this happening on additional files it can indicate a problem with your Internet connection.
Alternately you can also initiate a direct download by following the steps listed in http://forums.adobe.com/thread/981369.