Call the # above to list on the flight. You can get a seat request pass from the kiosk then...you've gotta have the confirmation number to enter into the kiosk, though...very important!
Best to get to the gate before the crew to get the paper j/s pass the CAs and some FOs carry around...intercept them as they arrive. I've had one CA not have any, and the gate refused to let me ride w/o one. Luckily, I got a spare from another CA from another flight.
The gate agents have historically been weak on j/s procedures and where your reservation is in their system. It's imperative you have called and listed before, and received a record locater number. You'll usually show up on their list as FD[last name] and that you cannot ride the j/s and need a seat in the back.
If the CA is cool, he'll ask the gate to put you in 1st if there's room. Otherwise, they'll assign you one in the back. That's when I wait 'till everyone has boarded and ask the lead FA on the plane if it's OK to occupy a seat up front. I haven't been denied yet....
Hope this helps. The AC crews have been great about this, but there are one or two that have been a pain. But, I'm sure we have the same types where we fly as well, eh?