Could be any number of problems.
Are you able to play JK games in other places besides the Zone? (ie: on IP)
If not, then the problem could be with your ISP or ports.
Otherwise, make sure you have the latest MSN Gaming Zone cookies, plugin's, patches, etc installed.
Make sure you have a full install of JK with the CD in the drive when you join.
What color is the host? If they are black or red, that could be your problem.
Can you host your own game and others join?
Also, just for kicks, are you behind a firewall? What is your connection speed?
Also, try this: go to http://qtracker.stomped.com
and download the latest version of Qtracker, and try using that to host/join games. If that doesn't work, let me know.
[This message has been edited by Kurgan (edited May 30, 2000).]