on windows vista, win7 and i think probability win8 you will need to do this:-
1) go to C:Program FilesFinal Fantasy VIIDriver Key Mod Pack
2) right click on the one you need to use go to edit
3) try below if you are on window vista or above
4) copy and paste below over the old hkey
[HKEY_LOCAL_MACHINESOFTWAREWow6432NodeSquare Soft, Inc.Final Fantasy VII]
5) it should now look similar to this
Windows Registry Editor Version 5.00
Modify the "DataDrive" value to point to your virtual CD / DVD ROM drive, for example "f:".
[HKEY_LOCAL_MACHINESOFTWAREWow6432NodeSquare Soft, Inc.Final Fantasy VII]
"DataDrive"="G:"
6) mount the image as it says in read-me and your finished