I think I ran into a similar issue as you. If you're doing what I think you're doing, you have just your CC3D connected to the receiver, probably powered via USB. In order for the flight controller to come online, it needs to be powered by something other than the USB (a battery, power supply, etc). When it does that, then your receiver should turn on, at which point you'll need to follow your radio's instructions for syncing. Then you should be able to test it out in LP! Good luck!