Usually when nothing appears it means that MeshCAM was not able to find a valid position to put it in. Usually this means that the support would pass under or over the part without intersecting.
Earlier versions were much more fragile with regard to support placement than the latest beta releases available in the news section of the GRZ Software website. What version are you running?
-Robert
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)