Open it in partmasterCAD.
then click on the "NC" button.
right click on the drawing and chose contour fromthe list.
Do not chose auto complete. (If the lip had it's own layer, you could use auto complete but not with the lip and housing combined.)
You will need ro make two contours.
One of the rocker housing including the lip, and one of just the housing above the lip.
When you go to Machining, you can chose the contour you wish to machine and the depth of cut leaving materialfor your lip.
It's late tonight, but if you don't get it I'll walk you through like I did Speedy.