There are two different ways depending on Bluetooth module that you are using.

(Old type Bluetooth module) If its serial number is smaller than 0640001 or self print shows firmware version 1.xx, enter the last 4 digits of "Address" in numbers or lower case characters.
Example Address: 00 0A D2 10 1F FD => PIN code "1ffd"

(New type Bluetooth module) If the serial number is larger than 0640001 or self print shows firmware version 2.xx, enter the last 4 digits of "Address" in numbers or upper case characters.
Example Address: 00 01 90 E1 A2 CF => PIN code "A2CF" With new type Bluetooth module, you may be required to enter PIN code every time when you connect from PDA to the printer.
This is because CMP-10BT does not save the information of connected Bluetooth module.
To change the setting to save the connected Bluetooth module information (Pairing Information), please refer to user's manual.