well i did mine on two circs so i didn't need the one for the middle of the thumb (since the middle was the divide between the two circs) The other markers go on either end of the thumb. I am not sure how many needles you are on right now but basically your thumb had something like 16 stitches on it right? When i divided mine i put them on the two needles so i had 8 stitches of the thumb on one needle and 8 on the other. the marker goes right between the palm of the hand and the thumb. (does that make sense?) also, there are tutorials and things on the pattern creator's forum here.
i know there is discussion of the thumb attachment there.