Hello barry and Welcome!!
Your Mac should come with a program called iChat. iChat works with both .Mac and AIM. Go to your applications folder --> Open iChat. If it is the first time that it has been opened, then enter in your AIM screen name and password and you should be all set to go. If you don't see a setup wizard when you open iChat, then go to iChat Preferences and you can set up your AIM on that manually. You do NOT need to download AIM. I hope this helps. Let us know if you need further assistance.
Cheerio
Searay
***EDIT*** I should read more carefully. Go to aim.com and create your screen name and password first! Only create the screen name and password - don't download AIM. Then open iChat and follow the above instructions.