If you apply Displacement to a non-displaceable object, like an imported (external) object, it will not show up in the 3D view, and in order for it to render properly you need to change its rendering method in the object's Rendering tab to "Force Displacement". Results can be difficult to adjust and depend very much on object geometry density. Be sure to use very small values for displacement at first, and increase in small increments, or you will likely "explode" your object's geometry.
- Oshyan