Hi,
You need to go to the Annoyances tab and assign a hotkey to Join waiting list, then you need to go to the Poker lobby and click on a table and then click Join waiting list hotkey, it will not do it automatically for you but once you click Join wait list hotkey then when a seat becomes available you will be seated and the table will auto open.
See this FAQ for Annoyances tab - Hold'Em Manager 2 FAQs - Poker Software :: Annoyances (TN2)