Your thumb strap isn't right. Also, you may be holding your hand in the wrong position. If you have one of Marc Savoy's videos he explains the way to use your thumb. One should be pushing with the palm of the right hand, really relieves the pressure on the thumb and makes it easier to move around on the keyboard. On the high notes (closest to the floor) one if forced to use one's thumb to push but on the middle and low notes it is easier to use your palm.

I'm confused. If I press my right palm, or any part of it, against the keyboard, my fingers are curled up like they've been in a fire.

Link #2 shows some nice shots of Marc's right hand up close, and he isn't touching the keyboard with his palm.

I would welcome something this simple to improve my playing, but I just can't find the position that you (or Marc) has described.

Is there something that I'm missing here?

thanks Hugh for the tip about the palm.
Nedro, If I'm right, what Hugh means is that you don't actually place the palm up against the box, but use it to give some back-up force to the whole right hand in general, including the thumb. correct me if I'm wrong Hugh.
both you guys have cool websites too!

Here's a freebie from Ganey Arsement. I haven't seen Marc Savoy's lessons, so this is what I've used for reference. See link #2.

Ganey is on the forum, and can play the buttons off an accordion.