When you work the dividing round, you cast on 2sts which will help ease the opening for the thumb. Now you're going to work the thumb sts and you need to pick up 2sts where you cast on the 2sts on the dividing round. When you do that you'll have those 2sts on your right needle with the working end of yarn dangling at the end of the right needle. Place a marker and start knitting the sts off the first dpn and continue around. As you knit around, you'll knit the 2 cast on sts and reach the marker for the end of round one. Here's a video
for picking up sts that may help.