In my opinion the easiest way to put a toolpath on that face is just simple 2-d pocket.
1. switch to a color that has not been used yet.
2. create curves all edges of the face you are cutting
3. Select the pocket toolpath
4. when you get to the chain manager, use window mode and also turn on the color mask to whatever you set in step 1 then draw a window over the whole part. Chaining done !
This generates a toolpath very quickly.
Now if you really want to use the horizontal area why don't you post a Mastercam .Z2G file with your machine definition and tool library already loaded in the file and make your best attempt at the horizontal toolpath. From there it should be easy to redirect you.
When you did try it did you use depth limits ? You have to remember that that toolpath will automatically try to cut every flat area on your part unless you find a way to restrict it.