There's a video for picking up sts on the Tips page, and it's basically the same. The way I pick up the sts from the cast on at the underarm is by sliding the needle so the sleeve sts are on my right hand needle like I'm at the end of a RS row, and then pick up those sts from the cast on. I usually add a couple on either end of the cast on by doing a M1 so that helps snug up the gap. Then knit the first round of the sleeve sts, decreasing the extra ones I made.
sue- knitting heretic