The SHADER-SETUP
(using Custom MR-Nodes)

Create new PHONG-Shader (MAYA defalut one)
and apply the following settings !

if you are done click the Node-button to get to the MR-Shader Setup
(the blue marked above)

•••

Assing a 'PARTI-VOLUME' Node to the MR Volume Shader-Node

Make this settings:

Scatter: 100% RED ... this one controls the Subsurface color.
Extiction: 0.500 ...( I call it : Caustic - Visibility - Attenuation)
this controls the strength of the Caustics
when they hit the Volume
0.1 is HIGH - 1.0 is LOW

Min_Step_Len : 0.02
Max_step_Len : 0.20
Controls the accuracy
(Don't forget this both !!!)

That's all for the PARTI_VOLUME Shader !

Go back to the Mental-Ray Custom-Shader Tab and apply
a PARTI_VOLUME_PHOTON Node to the PHOTon Volume Shader.

Scatter: round about 20% of the slider !
Extiction: 1.500
Min_Step_Len : 0.02
Max_step_Len : 0.20

So your Shader have to look like this :
(next page)