ATP will try to work with you to place you where you want to work. You can do this one of two ways, normally;
-Wait in the office until a position opens up at the location where you want to work. This can take some time, depending on the situation there, how long until someone who's currently instructing there has left before they get hired by an airline, and so on.
-Or accept a position somewhere else first, then hope ATP will transfer you later. They normally make no promise about this, but most of the time it works out well, especially if you have a family near a certain location.
I worked in Las Vegas for 2.5 months first, for instance, even if I wanted to go to Riverside. But I'm in Riverside now, living at home with my wife and commuting to the airport.
Cheers,
Helge