Make sure Dolby Dimension is active (Source Lights are cycling), and go into the Bluetooth menu of your desired Source Device. Select Dolby Dimension from the list of available devices. Dolby Dimension should connect.
If you would like to map this device to one of the three Source Buttons, visit the "Manage Devices" page in the mobile app and move the desired device to one of the 3 source locations. This will allow you to connect to this device by pressing the corresponding source button on Dolby Dimension in the future.