First, you should search for DiskImageMounter.app on your hard disk. On normal installations of OS X 10.3 (Panther) and later, it will be located in /Library/CoreServices/. It it's not there, you'll have to somehow get a replacement (reinstalling OS X is usually the easiest method).

By the way, Stuffit Expander won't work. It can open a variety of archives, but it cannot mount Mac disk images (dmg). There are some Windows utilities which can extract dmg files too, if you're interested in taking that route.

First see if you have the DiskImage Mounter on your computer:
HD-Systems-Library-Core Services-DiskImage Mounter

Not sure how that is installed but think it is part of the original Operating System. If that is the case then reinstall the OS & updates. Do you backup your computer HD? If you do not then you should as anytime you start making changes there is danger of losing data.

This is to Johnlh been trying to work this out and you helped me find this diskimagemounter.app thingy ur the only one that has helped and even the vapire administratoe thing couldn't help i have been looking for a answer to tis and read urs except i tried to get the trial but i didn't have a pic or whateva yeah i actually found the diskimagemounter.app but what do i do with it any answers for that? Do i open it? I don't know a lot about imac's but im not sure if i have to open it up and if i was to what do i do from then on please?
Thanks Heaps Johnlh i give u heaps more respect than vampire administrator cause he wrote it so complicated. Kudos to you and reputation to you i wished i could make ur rep. go much sky rocket than him becasue u really helped me.
Thanks again.
Rezo12345