The problem with Myidtravel is that as the time for the flight approaches, it will lock you out with a "general booking error". WN uses the same thing and I had them leave me at the gate because I couldn't list 30 prior and the agents were no help. I have heard that if this happens, list for the flight the next day and they can move you onto the current one. You just need to be in the system. Haven't confirmed this yet however.
As for AA when I get general booking error, I have had a few gate agents do it at the gate but the service center seems to be a good option as well. I just tell them I tried to list and got the error. I show them the screenshot and then they tend to help.