i have the function "mentalrayForMaya_v15". after installing mr, and rebooting (you never know about windows). you should be able to enable mental ray with the plugin manager. maybe you had maya still open when installing mr? or did not restart?
another thing could be that you use PLE 4.5 - as mr does not work with PLE.
but real nice of mr 1.5 to read hdr images - the beta could not