Using El Capitan, why can't I install some dmg files I recently upgraded to El Capitan, and now I am finding I can't install certain software. For example, I needed to view WebEx. WebEx had me download a dmg, but when I clicked it to install, it opened briefly then failed silently.

Then, my Adobe Photoshop and Dreamweaver told me I needed to restore Java 6 (even though I have Java 8 installed and it is backward compatible). I downloaded the correct Java 6 dmg, but again when I tried to install it aborted silently.

I tried launching the dmg from the command line, but that doesn't seem to be recognized as an executable.

FYI, I had selected under security "Allow apps downloaded from Anywhere"
You are opening a DMG (Disk Image) file that is mounting a volume which contains the installer package to run.  To see mounted volumes on the desktop, go to 


  Finder > Preferences... > General


and check the External Disks checkbox.