all you have to do is go in the Hypershade, select your base misss_fast_skin shader, and display "output & input connections". If you set it correctly so far, there should be a misss_fast_lmap node within the lower pane. This is the one for which you'll have to define the target baked texture for the lightmap.
ps: if your lmap node doesn't appear, then you go to your misss_fast_skin attributes , ... twirl down till custom shader/lightmap shader attrib displays ... and target a mental ray misss_fast_lmap.
Hope this helps,