Move the troop transport onto a world that has a citizen you want to load onto it. Then click on the world to select it which will display the graphic shown below in the lower left-hand corner of your screen. The bottom tile on the right will cause the troop transport to come out of the world on your NEXT turn at which time you will be prompted to select the citizen to load onto the troop transport. I STRONGLY disagree with the design decision that makes you wait a turn to eject ships from a world instead of ejecting them immediately, but that is the way it works right now.
