Are you looking for CAD or CAM? CAD is a drawing programme, CAM takes a drawing and produces toolpaths from it then gives you G-Code.
There is a CAM programme that comes with Mach, its called LazyCAM and should be installed when you installed Mach. Its still Beta but may do what you want. Also if you look at the Links button at the top of the forum you should see some links to CAD and CAM programmes.
Hood