I used to play this game a lot when I was younger, but I don't think I ever actually managed to find Carmen Sandiego. Where the heck is she?
Carmen is always the very last suspect you catch.
The exact number of cases you have to solve before she shows up varies between the different versions of the game (I think), but once you chase her the first time, even if you don't catch her, every case after that will be Carmen.
The exact location she ends up at is random, just like for every other suspect!