Crashes When Loading A Save File/World

Whenever I try to create/load an existing world on single player, the game always crashes and I can't figure out what might be causing this. I've positive that this couldn't have been caused by the age of the mac because my friend could play with a much older mac. I got the game a week ago and have been super upset that I haven't been able to play it at all yet. If this could be resolved asap I would appreciate that.