In Colloquy, bring up the connections window (if it's not visible when you open it, in the menu bar, go to "Window" > "Connections").
In the New Connection window, click the down arrow next to details.
The text boxes:
    Nickname: your ADISC username
    Chat Server: adisc.org
    Chat Server Port: 6667 (or 7001 if you want SSL, and select "SSL Connection").
    Username: doesn't matter what you put here
    Server Password: your ADISC password
    Real Name: doesn't matter
Join Rooms: You can add any channels you want here. The first one you'll of course want is #adisc.  Click the + and type "#adisc" in the text box.
You can also check "Remember this connection" if you don't want to have to redo this all over again.