Get the OP Enhancement Package v5.1 to get the latest material for online play, including the Directory server fix, then log into Dynaverse and pick a server.. make sure to read the server descriptions for any additional downloads you need to join in..
Major campaigns are usually announced in this forum board.. no major campaigns are currently running, but some are ijn development.
also, with the Pack, you should be able to play any Instant AAction game on Gamespy Arcade (firewall configured correctly of course)
Anyhow, I hope this helps.