The character should appear in the character list regardless. He would just not be allowed to log in to the character under certain circumstances (having logged out in a SB or ShB area and trying to log in on a platform that does not have SB or ShB registered). Make sure both the Mac license and Steam license are registered to the same Service Account. It's possible to have multiple Service Accounts with separate licenses rather than a single Service Account with multiple licenses, and if a character doesn't show up at all after registering a new platform, this is usually the problem.