Apple Magic Mouse is discoverable, but not pairing
My issue was that my Magic Mouse was discoverable through the Bluetooth Preferences pane, but when I clicked 'Pair', it couldn't connect. After an hour, I found a solution that worked:


Turn off the Magic Mouse
Open the Bluetooth Preferences
Delete your mouse's profile
Be sure the Bluetooth Prefs pane is searching (indicated by the spinner)
Hold the mouse button down, then flip the switch to turn it on (it should blink green during this time
While still holding down the mouse button, click 'Pair' in the Bluetooth Prefs pane