For some reason, this didn't work with the standard GUI bluetooth tools, so I did the following
- Run the command-line bluetoothctl tool
- type "agent on"
- type "scan on"
- press the button on your keyboard, the light should flash if it's not paired with anything else
- type "devices" (this shows the list of devices)
- type "trust xxxx" where xxxx is the keyboard MAC
- type "connect xxxx" where xxxx is the keyboard MAC
- type "pair xxxx" where xxxx is the keyboard MAC
- it should give you a code, type the code on the bluetooth keyboard's numbers, then press enter (on the bluetooth keyboard)
- hopefully you should be paired now
Note: I wrote this from memory, and I'm not sure if the trust/connect steps are necessary.
See also: https://wiki.archlinux.org/index.php/Bluetooth_Keyboard#Pairing_process
thank you!