For those working with Maya 2011 there is a good tutorial on how to get MetaSL shaders to work with the mental ray renderer. Now you can author your shader in mental mill with the help of the graphical debugger and the mental ray preview plugin.
Just follow the link and you will be guided through the necessary steps how to set up everything so that it works smoothly.
There are a few things that you need to take care of in order to have a smooth experience. I will report them in a seperate post what you need to take care of.