According to
http://www.nwa.com/offers/alaskaupgrade09/, you have to request the upgrade at the gate. Notice also that the flight must be Alaska-coded, and not a NW codeshare.
Beginning March 1, 2009, WorldPerks members who have earned Platinum Elite or Gold Elite status for the 2009 program year are eligible to receive unlimited complimentary, space available upgrades on Alaska Airlines-operated flights.
There are two ways to request a First Class upgrade when traveling on Alaska Airlines:
Full fare ticket upgrades may be confirmed in advance of your travel date:
If you have purchased a full fare ticket on Alaska Airlines (Y or YAS fares), you may call Alaska reservations directly at 1-800-252-7522. Advise the agent you are a WorldPerks Platinum or Gold Elite member with a full fare booking and that you would like to request an upgrade. Upgrades are subject to availability.
Discounted fare ticket upgrades may be requested at the airport:
When traveling on any fare type, simply present your WorldPerks Platinum Elite or Gold Elite card at any Alaska Airlines gate or check-in counter and request to be placed on the First Class upgrade waitlist.