I havent used that specific DVD, but finding the chord tones means to look at the chord underlying the solo, and notice when the specific notes from that chord are being played = so, for a major chord, notice when the root, the 3rd or the 5th are being played by the soloist.
Justin talks about the same thing in the Master the Major Scale DVD, which Im currently doing, and he says that as you play the scale you should notice when you hit one of these scale tones. At first I didnt get it, but since Ive started doing it the way the scale fits around the chord shape in each of the positions becomes much more obvious.
So, if he says to do it, I reckon doing it will probably lead you towards noticing something worth noticing.