Compsoc runs a Jabber server with conferencing. If you haven't run into Jabber before, it's an instant messaging protocol with all the stupid, broken features of AIM, MSN and the others, but with an open specification. Honestly, you really want to use IRC instead.
Anyway, the most important thing you need to know is that the name of the server is:
jabber.compsoc.kent.ac.uk
If you get that wrong, then it won't work at all (the server knows its own name, and will complain if your client thinks it's connecting to something else). The server's actually running on compsoc1, but forget I told you that; if you even hint to your client that it's meant to be using compsoc1, then Pain Will Ensue.
Your client should be able to create you a new account when you first use it. I had to poke around for a while to find a Linux client that'd create accounts; I ended up using Gossip. Gaim is a bit more capable as a client to actually use, though.
If, like me, you don't live on campus, then you probably want to know how to use Jabber down an ssh tunnel. The general idea is the same as IRC forwarding: a port on localhost gets forwarded by an ssh client to the Jabber port on jabber.compsoc.kent.ac.uk. However, the hostname problem above comes up again -- you can't just tell your Jabber client to use localhost as the server, because then you won't be able to connect to the server. Instead, you must be using a client that lets you set the server's name and machine-to-connect-to separately -- Gaim does this (as does gnugadu, I think?).
Anyway, you'll need to set up an ssh tunnel, by doing something like:
ssh -N -C -L 5222:jabber.compsoc.kent.ac.uk:5222 xyz9@raptor.kent.ac.uk
Then in Gaim's "modify account" window, put "jabber.compsoc.kent.ac.uk" in the Server box, click "Show more options", then put "localhost" in the "Connect server" box. Then it'll probably work.