Colloquy is an IRC chat client for OS X. It can be downloaded here.
Click "New" in the "Connections" window:
In the "New Connection" window that appears, fill in the following options:
Nickname: your forum username
Server Protocol: IRC (Internet Relay Chat)
Chat Server: irc.adisc.org
Remember this connection: checked
Chat Server Port: 6667 (or 7001 and check SSL Connection for an encrypted connection)
Username: anything you like (keep in mind that people can see this)
Server Password: your forum password
Real Name: anything you like (keep in mind that people can see this)
The "adisc" window should appear. In the left column, the channels you’ve joined are shown and the chat is found to the right. Settings such as appearance and logging can be set from the menu "Colloquy > Preferences…"
Once everything is setup, you can connect again by clicking the adisc entry in the list in the "Connections" window, and then clicking the "Connect" button.
Most standard IRC commands work in Colloquy, as well as some special ones documented here.