If you need caustics interaction with the mip_matteshadow shader, connect a new mia_material_x (the defaults are fine) or a mila shader into the mip_matteshadow Shader Group (shadingEngine) Photon Shader slot.
In mip_matteshadow you have to use the Catch Indirect feature (don’t forget to set the Indirect slider to white).
Of course you need a Caustics Photon emitter and you have to enable Caustics in mental ray Advanced Settings.
Caustics with GI Next is supported in mental ray 3.14 beta.
In mental ray 3.13 (mental ray for Maya 2016) you have to use other system for indirect lights (for example final gathering).