To use the Maxwell grass primitive in Maya, select one or more polygon meshes and click on the 
shelf button. This will attach one grass object to each of the meshes. The grass objects generate grass blades on the surface of their base meshes.
You can apply a material to a grass object in the same way as for any other renderable object. Note, however, that if you click on the grass in the viewport, Maya will select both the base mesh and the grass, so if you try to apply a material to the selection, you will also apply it to the mesh. Use Outliner or Hypergraph to expand the object group and select only the grass shape (make sure they are set to display shapes).
There are no Maya-specific attributes for the grass object. All the parameters are explained on this page of the main Maxwell manual. You should also visit the Grass Examples section in the knowledge base for tips and starting points.