I assume you are not talking about EC seats here as, unless you are a Delta Elite, you will have to buy them. As for Preferred seating, I just booked an exit row today with no problem. You must make sure that the system took your MVPG number and shows that you are an Elite on your reservation. It should then allow you to choose a preferred seat as per the agreement between Alaska and Delta. Try going back in after your original session and see if you can do it a second time and noting if you can see your status under "passenger" as "Gold Medallion/Elite Plus" on the reservation page. If you can't, clink on "change frequent flyer number" and enter it again. You should then be able to pick a PS. Hope this helps.