Shoot the trigger and It will activate. I don't know about this, but you could try luring the rat over there using the sneaky squeaker. --- The best dressed person and the official Doublade master of the Pokemon X and Y board! My Friend Code: 1719-3885-3128, my In game name Is Aaron.
I think you can also try climbing some electrical wiring and then shoot a valve or wire near the mouse to send it running towards the mouse trap. --- I like how the GameFAQs spellcheck shows that the word "Wii" is fine, but the words "Xbox" and "Kinect" show up as typos.