Can you mount them manually? Disk Utility can also mount dmg's.

Funny you should say this, my original account became corrupted so I am now using alternative ones. And for some reason the dmg files are not automatically mounting. I can't find anything in prefs in system, finder, disk copy or browser to control this. So I'm a little confounded too. It only happened after I reinstalled OSX, then set up new accounts & now also use one of the earlier test accounts.