The description of your Bluetooth GPS
The HI-408BT Bluetooth GPS receiver outputs standard NMEA over Bluetooth SPP.
So we need more details:
Operating System - Windows? Version of OpenCPN
just in case?
When you start the mouse, does a new COM port pop-up (--> device manager )?
Did you select this COM port with a baud rate of 4800 at the communication settings of OpenCPN (Apply/ok)?
Are you sure that no other program is using this port at the same time?
(Did you study the excellent manual of OpenCPN?)