Sorry to hear that you had a frustrating experience.

If you want to change your seat assignment and it's less than 24 hours before your flight (which I'm assuming is the case since you said you've already checked in), you can only do this from the check-in page, not your reservation page. To change your seat after you've already checked in:

(1) Start the check in process again

(2) The check-in website will say that you've already checked in, but it will display your itinerary in a box in the middle of the screen.

(3) Your current seat assignment(s) is (are) listed in blue, underlined font next to the flight(s). It's blue and underlined because it's a link. Click on the seat assignment you want to change.

(4) You will be taken to a new screen with a seat map. Click on the seat you want, and then click "Submit" (or "Continue"... I can't remember the exact details...)

(5) After you've submitted your new seat assignment, you'll be taken back to the "original" screen with your itinerary in a box in the middle, along with flight info and your (new) seat assignment(s).


A few other tips:
*If you don't get the assistance you want from any customer service representative, hang up, take a deep breath, and call again. There are some bad apples, but most representatives are quite helpful.
*If you have a bad experience, fire off an e-mail to customer care with all the info. The powers that be can't do anything about it if we don't tell them.
*If you have a good experience, fire off an e-mail about that as well. Even better, here's what I do if I've had great service on the phone: I tell the representative that they've been very helpful and I'd like to be transferred to a supervisor to share the good news. Positive reinforcement can't hurt!