XChat is an IRC client for Windows, OS X, and Linux. There are multiple builds maintained by different developers.
The official version costs $20 and can be found here. Major free versions include XChat-WDK and Silverex (XChat 2). A Mac version, XChat Azure is available for free here.
Open the Network List. This can be found by going to the XChat menu, then selecting Network List, or pressing Ctrl + S. It should bring up a screen like so:
Fill in the nick name field with your ADISC forum username, and fill in the user name and real name fields however you like.
Click the Add button. This will cause a new network to appear, entitled New Network. Type in "ADISC" for the network name, then click edit. This will cause the following window to appear:
The "Use Global User Information" checkbox should be ticked. Enter your ADISC password in the "Server password" field.
Click the box saying newserver/6667. It should now allow you to edit the contents. Windows users may need to hit the edit button. Replace "newserver/6667" with "adisc.org/6667" and press enter. If you do not press enter, your settings will not be saved.
If you intend to browse with SSL, then instead, replace "newserver/6667" with "adisc.org/7001". Then go to the bottom half of the screen and tick the two boxes marked "Use SSL for all the servers on this network" and "Accept invalid SSL certificate". If you get an SSL error, you most likely have not ticked the second box.
At the end of this, your window should look like the left image if you are browsing without SSL and it should look like the right image if you are browsing with SSL:
Close the network list, click connect from the Server drop-down menu, and begin chatting.