Has it ever opened since you got it? It could be anything from the latch not working properly to the door being so badly out of alignment that it's jammed shut.
Don't pull on it from the outside. From the inside, pull the door handle and bump the door real hard with your shoulder. If it still won't open get out your tools and get in there and figure it out.
Oh, and welcome to the wonderful world of old trucks... and stovebolt.com
