What is the "FmKit"?
It is a radio transmitter that has a small computer controlling it. It is designed to broadcast a signal (a tone code) on a specified frequency. It will send the tone code either after a set amount of time from being powered on, or after it stops "hearing" your aircraft, and it continues to receive power. The 2nd option is called "Vox" mode, and is preferred by most. Which mode the FmKit uses in user programmable along with the frequency that the radio uses to broadcast on.
Ok, the aircraft is missing, how does the FmKit that is on the lost aircraft help; What do you need to do?
You need a radio receiver that can "listen" for that tone code on the frequency that it is being broadcast on. And, it is helpful, if the receiver you use has a directional antenna. And, depending upon the distance to the FmKit, an antenna that increases gain.
What frequency should it be on?
The best frequencies to use are those that are:
A, Legal for you to use in your area.
B, Not crowded by other equipment using that frequency.
C. Programmed or able to be programmed into the radio you have or plan to get.