Odd, if I log in with my Apps account as primary and try to go to G+, I still get the message about Profiles not being enabled. I guess they're just testing on a few domains, or maybe rolled something out by accident.
The trick is to be logged into Gmail and G+ with your Google Account, log off from Gmail, log back in with your Apps account (one which has been transitioned to a Google account) and finally switch to G+.
