How do you install this sensor on the exterior doors. These door's swing in so there is no door casings to put them on.

These Entry Sensors apply onto any surface with adhesive - one piece on the door, and another piece on the frame (it doesn't matter which one goes where). If there's not enough space, you can give our Support team a call at 800-548-9508 and we can get you a Slim Magnet, which might help.