That option is grayed out until you have 3D geometry in your part because obviously you can't do a cut when there's nothing there to cut from. Once you have something modeled, then you have the option to choose to do an add or cut.
I do have a 3D geometry that I'm trying to extrude holes through, however, the Add/Cut button is grayed out as you can see below. This just started happening today. In the past I was able to draw a sketch on a surface and use the drawn pattern to remove or add material.