The Scanner appears whenever you place a trap. It shows all the Monsters that are trappable in that location.
On the left of the Scanner is a luck monitor. The more luck you have the higher the gold in the monitor will be.
Click on "Choose Bait" to fill your trap with Monster Bait
Click on "Place Trap" to put your trap on the ground or "Pick Up" if you have changed your mind
Click on "History" to see the last 20 or so monsters that you have caught
Once you have set your trap clicking on it will re-open the scanner