I would say download and install either [urlhttp://download.adium.im/Adium_1.4.dmg]Adium 1.4 (DMG File)[/url] or
Adium 1.3.10 (DMG File). It
should just be a case of installing an old version, it it complains just delete Adium.app and install, all of the account specific stuff is in ~/Library/Application Support/Adium 2.0
EDIT: Just to say my Pro is on 1.4.1 with no problems, Snow Leopard is all up to date too (bar the latest Safari update...)