Winning Poker Network,( Black Chip, Americas Cardroom, etc.) do not support setting the seat position as a preference
As frankace12 says you can right click and select 'seat me here' (or something similar) every time you get to a new table, but you cannot set it anywhere in the options
This is, in fact, the reason that the Winning poker network is not supported by HEM, and does not have full functionality (when multi-tabling) on PokerTracker