Hi Carpenter - MoI is expecting for you to select a curve for the rails, not a surface.

In the example you gave, it looked like you were selecting the surface as a rail.

To make it work, do a second click on the surface to select an edge of the surface - an edge can be used as a rail.

In the future I want to make this a little bit easier, by trying to screen out other types of objects that are not valid for the current operation. Like for example it would be better in sweep if MoI only allowed you to select edges or curves at that point, since those are the only valid objects for that operation.

Please let me know if you need any additional information for this.

- Michael