as some have already said, it is a simple matter of a flick of the wrist with a small flathead screwdriver or knife. at least with master locks. I wouldn't try matching up keys, as I have some numbered from 130 all the way past 600.
As the one fellow said, locksmithing is a trade. But this is not one of those times a trade is required unless you don't feel like doing it yourself.
You may be best to goto the store and buy another of the same lock. Look at it, study it and you should be able to figure out how to pop it open. Then do the same with the one on the gun. I myself used a small flat head screw driver instead of a knife cause the shaft wouldn't hurt anything if I slipped. This way you also have a replacement lock. Unless you want to goto combination.
note:
1)rubber on the masterlocks gives enough room to squeeze a small screwdriver in and turn the head so the lever comes away from the teeth.
2)I think trigger locks are more of a deterint then an actual lock.
3) as the one fellow said, sometimes the trigger guard can come off, or in my situation, just open the lever and it will fall off. (talking about the lever, does anyone know how to use a normal trigger lock on a lever action like a savage 99?)