It sounds like the scope your friend loaned you was for a regular rifle, not an air rifle. If you really want a scope for your pumpmaster, check out the selection at Walmart. They have a variety of air rifle scopes that are fairly inexpensive. I believe most of them have the smaller rails that should fit your pumpmaster. Here is one example that I found online:
Walmart.com: Crosman Red Dot Sight: Optics
You can also find them at airgun depot and pyramyd air, and other places like that. Just be sure you get one to fit your pumpmaster dovetail rails (11mm, according to what I've read online).