I concur with SimX. I believe you do have to have @mac.com. I included it in my buddy list and its in the name when you instant message someone from your @mac.com name.
BTW, you can also message @mac.com people with the current aol beta. One problem I've noted is that when you use your mac.com name for AIM all of your buddies will not show as online and they will not be able to see you, because they are using third party versions of AIM or older versions. I've noted this is a big problem with my PC friends.