I just ran into this myself. At least in J Asiana assigned a default seat for the booking but if you want to change it online you need to have or create an Asiana account in the name of the passenger flying. I can retrieve a booking for another passenger from my Asiana account but I cannot assign or change a seat online if the name on the Asiana account does not match the name of the booking. If you cannot get it to work online then call.

Since you are looking to select your own seat all you need is to create an Asiana account in your name and retrieve the booking from there (if you can tolerate the horrible Asiana website operation).

Edit:
Also, make sure your Asiana FF # is in the booking before you attempt to change it online via Asiana's website.