I couldn't find a free version of the manual anywhere to help you with threading the bobbin, but I did find a few options to help you thread your Viking Huqvarna by yourself...
Youtube has lots of other sewers making videos about their Viking machines, and that will help you get to know your 6440 better. Here's a Hisquvarna Viking sewing machine threading guide.
This second video is a bit longer, but has lots of useful tips for Viking machines demonstrated on a model similar to yours.
If you are willing to spend a bit of money online, you can get the a printed or PDF version of the manual for the Husquvarna Viking 6440 from Littlepackage.com
Here you can purchase the service manual for many Viking machines, including yours with lots of troubleshooting help, repair tips and caring for your machine.
