Wait lists are shown in the software lobby, as well as on the tournament info screen for multi-table tournaments. If you are already on the wait list, in the lobby, the wait list is shown in order and will specifically say:
"All seats full. You are player X of X on the waiting list."

If you wish to join the Wait List, Click on the 'Join Event' button in the lobby. If the table is already full, you will automatically be added to the waiting list. You can go back to view the Wait List in the Lobby to see that your name is there.