Hi ncg1983, welcome to FlyerTalk! ^

If you know the United confirmation code for the booking (6 letters/numbers) you should be able to assign seats at United.com. Go to the itineraries and check in section and look for the "guest access" link. It will ask you for the confirmation code and your last name and then will take you to the booking where you should be able to assign seats. I've never had trouble doing this even, when I'm booked on a united flight via a codeshare, like you are.


Here's the page to start with:
http://www.united.com/page/middlepag...riesandcheckin