The mouse mouth has to be powered on first, and the bluetooth receiver has to be plugged in also, before you power up the PC. If it's still not recognized, you may have to do the device's simple signal sync procedure, which usually involves pressing a button or two, and watching the blinky lights.

(Also don't chew marshmallows or watch Star Trek while you try to sync, or type. Wink "Beam me up Snotty. I'm blue in the tooth." )