It appears that somehow Reader has become the default program to open DMG files. You'll want to open the DMG with DiskImageMounter.app. DiskImageMounter is found in /System/Library/CoreServices/DiskImageMounter.app.
- Right-click on the DMG installer file and select 'OpenWith'
- This will reveal a menu that should have DiskImageMounter.app listed.
- If DiskImageMounter.app is not listed, select Other... and navigate to /System/Library/CoreServices/DiskImageMounter.app.
- This will mount the DMG revealing the actual installer .app file inside
- Double-click the .app file and proceed with the instructions to install Flash Player.
You'll actually want to make DiskImageMounter.app the default program to open DMG files so that you don't encounter the same scenario in the future.
I found 'Library' ...eventually but I'm not sure how.
Unfortunately I am not computer literate, so what do you mean by your 2nd and 3rd sub bullet points? I have tried double-clicking on the Disk Image Mounter.app but nothing happens.
And, how do I make my DiskManagerMounter.app the default program to open GMG files?
Double-clicking the DiskImageMounter.app won't do anything by itself. You first need to right-click on the DMG that you want to open and then select Open With, then navigate to the location where DiskImageMounter.app is. After selecting DiskImageMounter.app, place a checkbox next to 'Always Open With' at the bottom of the window. This will make it the default app to open DMG files, as in this screenshot:
Many thanks for all your help, very much appreciated. Everything works fine now. Heaven alone only knows what I did to undo the app in the first place.