Just type for example 123 to computer and then the same passcode to phone, 123, it should work.
Once you get it working, you can change the passcode to be more unique if you feel that is needed for extra security.
If you like the computer and phone to connect more automatically everytime you put Bluetooth on in PC and phone, use the "set as authorised" option in phone when you connect the first time, it makes the usage easier.