You need to ensure that the geometry has enough polygons to capture the detail. You might need to subdivide or re-mesh the area of the model you want to add texture too. If you have a very large object you might want to alter the scale of the texture so it is suitable.

By default there isn't a vertex group selected. You need to add vertices to a vertex group and then type that name into 'Group Name' box. This is to give you control of where the texture is added.