As of now, you just install Gaim from http://gaim.sourceforge.net/downloads.php
Then, install OTR from http://www.cypherpunks.ca/otr/.
Then, if you're using the Gaim beta, you need to install the files form this zip: http://lists.cypherpunks.ca/pipermail/otr-users/2006-January/000531.html
Then, you probably have to restart the progroms and reboot half the world. Then, go to Tools> Plugins in Gaim, enable Off The Record messaging, and configure it. In Configu you need to generate a key.
If the party on the other end is also using OTR then your conversations can be encrypted.