Start the thumb increases just before the heavy red line in the mittern pattern. It looks like you work 24sts on the mitten chart, change to the thumb chart, increase between sts, knit the center st, increase and then continue with the mitten chart. The thumb chart is inserted at the red line and starts on row 6 with the green square with the M in it. You're rigth that "no stitch" means do nothing, skip that box.
Very good looking mittens. They'll be lovely.