Will this work on LED lights?

You cannot use the shunt repair function of this device on LED light strings, but the bulb tester and the open circuit detector will work. However, if all you have are LED lights, then I would recommend buying the model they make specifically for LED lights.

Pulling trigger didn't work. Voltage tester seemed to work but when I replaced the bulb that didn't beep it still didn't work???

You have to pull the trigger 4 or 5 times. also you need to check down the entire string, because more than one light may be out. I usually test the fuse in the plug part first, because if that is out, nothing will work. test your fuses, then run down the lights with the gun pressing the top button, it will "flash beep" until you get to the lights that aren't working. do this to the entire strand. then replace all those that it does not beep at. it is not a 5 second fix when you have a stand not working. take the 5 - 10 minutes to do all this and you will love this tool.

So... What does it actually do? I mean, exactly how does it work?

I purchased to use with my pre-lighted tree and tried again to use it on tree. I agree about how does it work?

will this work for a Martha Stewart pre-lite tree?

I have not tried it on a pre-lit tree but according to the instructions it is supposed to work on pre-lit trees too. It does work on strings of lights but if you have a string of lighted garland and you have several lights blows you might have to take some out of the socket and test them one at a time. The string of lights will usually still work with some burned out bulbs but when you reach a certain number of blown bulbs then the string will stop working. Good luck.