Hey, man, and welcome to the Jazz Jack Rabbit community.
First of all, here you can download patches for the game.
Then, here you can download various levels.
And, finally, here you can see what games are online - they are the games you can join.

If you cannot host\join other games, refer to these topics:
http://www.jazz2online.com/jcf/showthread.php?t=9102
http://www.jazz2online.com/jcf/showthread.php?t=10564
Good luck!