Well, remember that Tom was the legitimate arm of the family. He had no problem getting meetings with people such as Woltz. He was not the muscle end of the business. I'm sure that he could've gotten an appointment with Frankie under the guise of attorney, or even as a friend, since Frankie seemed sincerely happy to see him. Once Frankie was "outed" at the trial (until that time, the Corleone Family thought he was dead), perhaps he was allowed visitors. He may have even welcomed the meeting with Tom, since he might have wanted to make a deal.