See also Overview of sculpting with maps.
To sculpt using a displacement map
The Sculpt Using Map window appears.
Click the browse icon
next to the Map option and browse to select the Ptex file or image file you want to apply as a displacement map.
This must be set to match the Vector Space used to produce the vector displacement map. (See Extract a vector displacement map.)
For all other map types, use the default Normal (regular displacement map) option.
The displacement information is stored on a separate sculpt layer and can be modified globally (by changing the Strength of the layer), or locally (by applying masking to the layer).