I booked when logged into airfrance.us, and was unable to change from my FB number over to my SkyMiles number (the option was grayed out). I called into the Diamond Line and they were able to:
1. See the reservation with my AF confirmation
2. Replace my FB number with my SM number
At this point I'm able to select seats and do pretty much everything you'd expect, contrary what other people are saying.
The only confusing thing at this point is that my "estimated points":
> Miles earned on this trip (estimated total provided for reference only) : 7 988 Miles
Given that LAX-CDG-LHR-CDG-LAX is ~12,000 miles, I am very confused where they are even getting that number.