Make sure you've got right-clicking enabled in your mouse preferences. If you can't right-click, control-click on the stencil. In ruler or guide mode your pencil should ride along the stencil.
Dear Ivy, Hello! and Thank You for asking this question!!! Best Wishes to you!!!!,,,,and thank you to Mike and Gekkou for the answers, this is a trick i did not know about,, again, the members always seem to help!!!