ZBrushCentral

displacement approximation question

Trying to render my displacement map from zbrush 3.5 on a low rez mesh in mental ray (maya 2011) (following the Scott Spencer tut) and I get the error:

// Warning: (Mayatomr.Geometry) : BASEMESH3:polySurfaceShape1: subdivision surface does not support separate displace approximation, ignored. Tune primary tessellation / approximation for displacement.

anyone else ever get this what does it mean?