You should not need to enter a PIN as it uses SSP (Secure SImple Pairing) by default . The important point is to press the RFD8500 trigger to complete the pairing process otherwise it will timeout :
Touch the RFD8500 in the list of available discoverable devices, and press the RFD8500 trigger to pair
when the Bluetooth LED starts flashing fast.
If this still does not work, try a factory reset of the RFD as follows :
Hold the Power and Bluetooth buttons simultaneously for five seconds, then release to initiate the factory reset process.