You must first add your players to the TGS system at your club level for the player to appear under association registration. Team admins can also add players to the team.
If you are a club admin you can select Club and players in your account on the left-hand navigation.
Select the Add button on the top right-hand corner. Once you have added the player and assigned them to a team they will show up in the list of players to register. The players must be a First Team player show on the registration list.