Ok, i know what's the problem! Maya is not loading the Mental Ray shaders and nodes! This is what i read at mental Ray's help files:
"mental ray for Maya supports many mental ray custom shaders. mental ray for Maya supports Maya-type shaders, textures and lights, as well as importing arbitrary libraries of mental ray custom shaders. These custom shaders can be assembled into shading networks using Maya's Hypershade and assigned to Maya objects. Integrated rendering with custom shading networks is therefore fully supported.
This is an advanced user feature. It is available in Hypershade only if you set the following environment variable to a value of 1.
...the problem is...
i don´t know how to do that
jesus loves you!