You should be able to either draw a path with After Effects' Pen tool, or import one from Illustrator and convert it to a shape layer. The path needs to have exactly three points and there should be no other shapes in the layer. Tangents will be ignored. Limber will use the first Stroke property it finds in the layer for the new bone. It will place the start controller at the first point in the path and the end controller at the last, so always draw your limb from the body outwards. After generating the bone, Limber turns off the visibility of the original path layer.