I found the visual way (so you can see what you are getting) is to start with the loaded the .gbr for the board top. Select this, then scroll down to 'non copper regions' and Generate Geometry. Now select this generated project and using view menu, 'disable all plots but this one'. Now use the edit menu 'Edit Geometry' and when it enters edit mode, use the draw rectangle tool to draw the slot you want on the pad needed. The again in the edit menu select 'Update Geometry'. Now select the non copper project again, scroll to 'Paint Area:' and fill in the tool diameter for the slot cut and Generate Geometry..click inside the slot rectangle you drew as above and it will generate a new project listing '..noncopper_paint' Select this and fill in the Create CNC job: (cut z to cut through the board, etc..you will probably want to run passes at half the tool diameter to avoid breaking the mill tool) and generate. This creates the CNC job object for the slot. Hope this is clear and works for you!!