Under Comm Links, click Add - Type: Serial, choose the BT serial port, baud rate, etc. Strangely, it allows this configuration, displaying the BT adapter in the list,ceven for BT adapters which have not yet been paired at the OS level. The pairing at the OS level allows you to enter the pin number required by the BT adapter, and I was under the impression that this had to happen forst otherwise no BT comms! But if this pairing didn't yet take place, I find it strange that QGroundControl still shows the (unpaired) BT device, and allows it to be added in the comms configuration. Not sure how it would expect a connection to succeed using a newly defined comms config to a BT device, if this hasn't yet been paired in the OS!