How are you trying to install it?
What rom are you running? CM7.1? Stock Rom? Version? (If stock, you have to be rooted.)
Are you flashing ClockworkMod Recovery from Rom Manager (top button)?
Find Superuser in your list of apps and delete the entry for Rom Manager (trashcan, right side). Open Rom Manager again and tap the "flash" button at the top, making sure you tap "allow" when the superuser prompt comes up.