So, how large is the file?
If it's small, it's the bittorrent thingy. I didn't use that myself.
If it's large (and it sounds like it, being a .dmg and all sounds familiar), then it sounds like the actual game. Make sure you've got an expander, like Stuffit Expander, that'll... well... expand it for you. The links around here worked, because I downloaded from here.