This is the closest thing I could find to a generic troubleshooting thread on the matter: https://deciphertools.com/blog/2017-10-02-no-mountable-file-systems/
Hope this helps, as ESC was a very enjoyable visual novel experience!
EDIT: Also, some people in other threads elsewhere suggested it might be tied to the version of macOS used to package the dmg vs. the version of macOS on your computer. I'm running 10.13.6 and was able to open the dmg; what version are you running?
EDIT#2: It looks like this might be caused by the dmg having been packaged using APFS (Apple's new filesystem), which would result in it not mounting on a Mac using HFS+ (Apple's old filesystem). If this is the cause, the options would be:
A. Upgrade your OS and ensure that you opt into APFS
B. Try this driver (at your own risk) that would allow you to mount APFS images in HFS+
C. Kindly request that Radical Dreamland repackage the dmg to use HFS+ so it is more broadly compatible with more Mac computers
That's what it was!! I have two Macs. My 2009 Mac Pro can only upgrade to El Capitan (OSX 10.11.6) but my MacBook Pro has High Sierra (OSX 10.13.6) and ESC opened up on the MBP! Thanks... don't know why I didn't think of that!?
*UPDATE: It works just fine on my 2009 Mac Pro too. Since I have them networked together in my music studio, I just dragged the ESC game into the applications folder on my Mac Pro (from the MBP) and it played without issue. The .dmg just wouldn't open on the older Mac...
