Hi and happy new year,
2 options :
1) Your board previously had a firmware uploaded - then you don't need to burn a bootloader into Atmega328, it is done once for good and USB should work. If you run windows on your computer maybe a fake "Trackball" or whatever it is named in device manager under "Mouse" appeared at Bluesaab's plug in. The best way to get rid of it it to Deactivate it, not uninstall otherwise you'll have to repeat the operation... Then you should be good to upload via USB.
2) Your board is virgin and then you must burn a bootloader into Atmega328 using ISP pins. I only own an Arduino Uno board and always burnt bootloader with it. Lot's of how-to are available to do it a search with "Arduino uno as ISP" should lead you into the right way.
When bootloader is burnt, get rid of the Uno board and connect BlueSaab to PC with USB cable. It would detect a new COM port and then you'll be ready to upload Bluesaab's firmware
Good luck !
01' 9.3 SE Conv - AS3 - Nexus 5 Android M 6.0.1 - BlueSaab v4.2 + Amp - Latest CDC repo.