its a really simple process....
1. you need a mounting program. I recommend daemon tools. You can get the lite version for free from here:
http://www.disk-tools.com/download/daemon
2. install the daemon tools software
3. right click on the icon in your system tray (right next to the clock)
4. from that menu click virtual cd/dvd rom drive > then go to your first device and click mount image.
5. then browse to the folder your image is located in
hope this helps

im not very good at explaining it lol
and by the way make sure that you have the pc version of your game
also daemon tools will open just about every type of image out there.
and there is the pro version in the general apps section too if you need it
if you need more help check out this better tutorial
http://www.youtube.com/watch?v=n8XRw9RmI_E