The Zombie Master IRC channel is located on QuakeNet. Use the following information in your irc client:

Server: irc.quakenet.org
Channel: #zombiemaster

There are many guides on using IRC all over the internet. The basic procedure is this: Start your IRC client, type /server irc.quakenet.org, type /join #zombiemaster, talk.

IRC Clients

The most popular client used to connect to IRC is mIRC. It's easy to use, works well, and is shareware. Some completely free alternatives are Trillian, ChatZilla and HydraIRC. There are plenty of other clients available and easy to find with a quick google search.