The NURBS primitives can be selected by the Create- NURBS Primitives command. The settings of the geometry can be adjusted in the option box or after creation in the channel box.
Note that the use of nurbs primitives is not that convenient. It's easier to edit and work with your geometry if you create it by using curves. However, in some cases it is convenient to use isoparms originating from primitives, in the section about selection masks it is shown how you can select those.