How do I use the Particle Disturbance Sensor?
Use the sensor like you would a weapon - put it in your hand, then click on it, the same way you would pick between snap, auto, and aimed shots.
Medkits are used the same way.
The other thing of note is how to use the display once you have it. It will show you all movement from anyone within an 8 square radius. It is best to use this before other people have moved. It will only show moving people, so if you have an alien hiding in a closet, it won't do much good.
Personally, I never could find much use for them, but perhaps others have.