Automatic: FAB 3000 will attempt to automatically detect the PCB border boundary and create a border layer.
Selective: Select one edge of the PCB border and FAB 3000 will attempt to close it. Should FAB 3000 get stuck it will prompt you to select the next edge (until it closes). A border layer will be created upon completion.
Go to menu: Fabrication / Generate Missing Layers / Border (Selective)