To use the net driver, you need to have db2jstrt running on the host machine. The default port then would be 6789, although you can specify a different port. I've read somewhere that you'd need db2jstrt running on the client with the same port as well -- I doubt this is the case, but try it if you get stuck. Also, the copy of db2java.zip has to be the same as the one on the server.
If you use the app driver, the client config assistant needs to know about the server, and the port that you'd use won't be the same as the above (usually 50000, if you've only got one instance of db2.)
By the way, the JDBC drivers on 6 and 7 are buggy, so get the latest fixpack (and don't forget to keep db2java.zip in sync.)
You might find a 3rd party driver, eg from
HiT Software much simpler to use.