Have you tried being patient? I can take 30-60 minutes from when you check the box (and save) in the management console before you can see it and manage it via the instance manager. Otherwise I'm guessing all the ports aren't open which you can confirm by trying to manage another PBX via instance manager. If you don't have another one in there then spin up one, free keys are free.
It might also help if you provided more information about your setup. If you are on an older version of 3CX this could also be the issue.