Yes, I had the UA confirmation number and the flight was showing up in my united.com account.

The problem is that once you've checked in (and any time after T-24, I think?) the only way to change seat assignments is through the UA check-in process -- there was a link to that on the "view/change seats" page for the FRA-IAD flight, but in this case following the link resulted in an error message directing me to use the LH site -- I suspect that was because the first flight (and hence the one I would check in for) was on LH.