On the navigators it will list a few specific times that there will be pin trading. Most of those times are just a cart that's brought out for sales of pins- very little trading.
Now, definitely go the time that says Officer Pin Trading. All the CMs will be out with lanyards on in the lobby so there will be lots of opportunities for trading.
We ended up passing by the lobby when they were just ending the Officer trading session. Kinda bummed about it. But now we know for next time to attend that specific session.