Once you do this, it seems very trivial, but it might not be the most intuitive thing for the inexperienced. This basically works for any IRC not in the default list.
Edit->Accounts and click to add a new IRC account (Choose IRC from the drop-down menu next to 'What kind of chat account do you have?'). Now click on the button below the label 'Which IRC network?' (usually it would say 'freenode').
Here you will have a list of available networks. Gnome will usually not be a part of it. Click on
Make sure that the server address is
irc.gnome.org. Try pressing
Edit to check again
Now you're set. Your account will look like the following
To join any room, click on