First, you must be using the latest Beta version of PAH.
Beta Release v1.19.6Once you are using the latest beta, you need to use the "Preferred Seating" option on PokerStars, then you configure which seat you chose via Options --> Preferences, under PokerStars (located beneath "Poker Sites") select the "Preferred Seating" tab. Then choose what table type you are using and which seat is your preferred seat.
PokerAce Hud cannot detect if you use the "Place Me Here" function in PokerStars, so if you switch your seat that way, PAH will not be able to detect who is where. But "Preferred Seating" is supported.