Your statement that you are fairly new to "this frequent flyer thing" leads me to believe that you are not a NW elite member. This complicates matters because you may have difficulty in getting a regular agent to find you availability. One must call WorldPerks reservations in order to upgrade the "M" bucket and there is often not upgrade availability for a particular flight (especially into BKK). Just keep calling and trying.

In the future, if you have date flexibility, call WorldPerks first and offer them the dates you want and ask them for upgrade availability and upgrade at the same time that you purchase the ticket. Sometimes you have to fly somewhat convoluted routings to get upgraded.

For example next week, I am flying PDX-SEA-HNL-NRT-BKK on a miles upgraded "M" fare booked in July. The return is BKK-NRT-LAX-PDX. The only portions not upgraded are those on AS ie: PDX-SEA and LAX-PDX.