At the edges of the thumb gusset, there is a purl st. When you come to that st in the increase row, purl it so that it is on the right hand needle. Now reach down with the left hand needle and pick up the loop of the purl st that is one row below that st on the RH needle. Knit that st through the back loop. There's a similar direction at the other edge of the gusset but in that case you pick up the purl loop below the st on the LH needle and knit it..
This gives you the nice little row of beads at the edges of the thumb gusset and increases at the same time. If you can't get it to work, you can ignore it and use your favorite increase at the markers.