Instructions, below, are for the Pidgin plugin, and I am not certain how much of that is mirrored in Adium.In other words, if they don't work for you, that's because it's not the same piece of software.
Due to some changes in the SSL handling, versions of Pidgin above 2.10.9 are advised.
Everything should work out of the box and once you are done you can start Pidgin and enter your account settings.
If you need to manually install the plugin for any reason: You can get the latest binary build here.
Bug reports for the Adium plugin should be posted to that thread.
Do not report issues already listed in the first post.
For more information about Pidgin, itself, please consult the official FAQ for the software.