In Windows go to the Control Panel or search for "Control Panel".
Go to: "Hardware and Sound" (click on the icon and turn your keyboard on and click on the connect button of the keyboard), click at "Hardware and Sound" on the text "Add a device"
In this field you will eventually see "Bluetooth 3.0 Keyboard".
Select this one and click on "Next".
Eventually click on the text "Or, try entering a passcode on it".
A passcode should now be generated, enter the passcode on your Bluetooth keyboard followed by the "Enter" button.
Might be you have to do this twice (due to timeout), in that case turn the keyboard off and on again and press the connect button on the keyboard and try the steps mentioned above again.