Did you make the booking through Air France? If so, your booking reference will be the one on the AF system. Unfortunately, I don't think the NW system can access it (unlike a KL reference which the NW system can see).

As you're flying on KLM, there may be a KLM reference which AF could give you. With that, it should be possible to access your booking through NW.

Hope this helps.

Simon

Edited to add...
I tried this last night with one of my own reservations made through AF. The KL reservation code does not work in the NW system. Sorry