R C-R if you go into the Node or Pen tool.  On the context toolbar the 5th icon in the Snap group.  Click on it and another set of icons will open up for construction and it also makes any existing construction marks available for snapping.  The first of the construction icons puts it into construction mode.  You'll be able to figure out the rest easily enough I think.  Read my earlier entry to Matt for a couple gotchas.  AD will ignore almost all other commands when construction mode is active so don't forget to exit construction mode when you aren't creating or editing construction marks.  Hope that helps.