Click on a transport and Psi will gather data.
Right-click on a transport to register. Registration involves entering your username and password for the given IM network. Once you do, Psi will connect to that network and retrieve your contacts. You can also add new contacts.
Once I had "registered" each of my IM accounts, I had a lot (hundreds?) of system messages.
Now exit Psi and fire-up Google Talk. You'll have all your IM contacts right there on your Google Talk contact list.
Once you've done so, you can chat with your friends on AIM, Yahoo, ICQ, MSN -- all using Google Talk. You can also add contacts directly within Google Talk. Just use the format [screenname]@[transport.dns.name]. For example, to add my MSN screen name you would enter email@example.com
What's the deal with ursine.ca? You can use any Jabber server you'd like -- you'll find a complete list here at Open Jabber Servers -- ursine.ca was just the server I used.
This process works, although in my experience, the MSN transport has not been reliable. One of my friends reported an weird "echo" effect.
As with any of these kinds of hacks, your mileage may vary. But you've got to admit -- it is very cool to be able to use Google Talk as a universal IM client.