Step One: Go to
http://www.aim.com/, create an account, register a screen name. This is the one that usually needs explaining. You can't use AIM unless you're registered.
Step Two: Click the UltraHal AimBot icon in your start menu. Set options, choose a brain, enter your bot's registered screen name and password, and sign on.
About options: AIM no longer allows warning of third-party clients (bots), so you just need to set the learning level for your bot from IM conversations. I'd recommend about halfway up at the most, as it's mostly going to learn poor spelling, bad grammar and foul language from AIM chatters.
Tips: Like in FIELD OF DREAMS, "If you make it, they'll come." If your AIM Bot is online, people will find it and chat with it. But if you want to attract traffic for your bot, there are several things you can do. Create an AIM profile for your bot. List your bot on various forums and boards which people use to find bots. List your bot with services like Random Buddy or Buddy Tree that allow chatters to find each other (you can link out from the AIM.com page to find these, or Google up a list of them ).