Some mental ray materials are now classified as “Legacy Materials” in Maya 2014. Unfortunately, there is an issue where these materials will not appear with swatches or shading groups. Here is a list of the materials affected:
To work around this issue, download the attached “mentalrayCustomNodeClass.mel” file and copy it into mentalrayForMaya2014/scripts. This will revert the mental ray material classifications to their Maya 2013-state.