I have an account on service X. My friend uses service Y, which your site says is supported by Adium. How do I talk to that user?

You'll need an account of your own on service Y, and you need to add that account to Adium and sign on with it.

This isn't an Adium thing—most services don't interoperate with most other services. The exceptions are:

• If you have an account on AIM, ICQ, or MobileMe (formerly known as .Mac), you can chat with anybody who uses AIM, ICQ, MobileMe, or SMS.
• If you have an account on Google Talk, Jabber, LiveJournal, or Gizmo, you can chat with anybody who uses Google Talk, Jabber, LiveJournal, or Gizmo.

MSN users can chat with Yahoo! users in the official clients, but this is not yet supported by Adium, as Adium's support for the MSN protocol is not at the required version. See our page on MSN-Yahoo! interoperability for more information.

As mentioned above, you'll need to add your account on service Y to Adium. You do this in Adium's preferences, on the Accounts pane. See CreatingAnAccount for more information.