I have no clue how crazy mac works, but I'd guess its similar to linux.
Open up a terminal, in Utilities - Terminal i think.
"cd /home/YourName/Downloads(Or where ever you put it)" after you've typed the first letter or two of a path, hit tab and it will auto complete.
This changes the working directory to the folder your .jar is in.
do "./File name here...jar" Type the ./ then the first letter or two and tap tab, and it should auto complete.
or maby "java file.jar" same thing for auto complete.
one of which should execute the jar file.
Minecrafts amazing btw.
^Thats the best site on the web.
אמא שלך, היא שמנה מאוד, יותר גדול היפופוטם.