So I just downloaded this mod, and it seems to work, but I need help with the commands. I'm not sure if I'm doing things wrong, but if I type "carsetcollisions" into the mafiacon, all I get is "cmd/cvar not found."

Did you load all th command files you got with mafiacon?  If not, then that's your problem.  I don't remember the exact commands to load them, but there should be a readme telling how to do it.