Installed game, file not working.
DiskImageMounter is the default application to mount .dmg images, so don’t worry about that.
You probably have a corrupted download, just try downloading the file again.
MacBook Pro 15" mid-2010 (i5, 330M, 8GB, SSD)
I had the same issue. The download kept getting stuck at about 600k of 25.8MB – it would timeout and an incomplete .dmg file would end up on my system causing the same error as noted above. It finally worked on the fourth try.
The relentless pursuit of mediocrity.