I found that if you...

1. log into your account on ba.com then
2. click on your name (upper right area),
3. select the dropdown menu "My bookings" then
4. select "My upcoming flights" then for a specific flight
5. click on "Manage My Booking" then look for the section Passenger details where you'll find your name with a greater than sign: >
6. click on Your Name there, which displays the Add Advance Passenger Information page: sadly, I had nice screen shot that would have shown you exactly what this looked like, but as a new user am prevented from showing it. In that list is the Known Travel Number
7. click on "View or change details"

You should be able to add the number on that screen (don't forget to save your changes).
I'm not sure if the number is only added to that flight specifically or adds to your account.