You don't really need a marker on the needles for this pattern and it's difficult to easily place a marker on dpns anyway. Use the yarn tail left from your cast on to mark the beginning of the round or, put a safety pin or a small piece of contrasting yarn through a stitch to mark the beginning of round if you'd like. Beginning of round is the first stitch that you knit joining the sts to knit in the round. Here's a video that may help. http://www.youtube.com/watch?v=9Ajp0QUWYkk
Cute sock pattern btw!