To import a surface to ANSYS in the Design Modeler, you will first need to create just the surface using a CAD program, such as Solidworks.

In Solidworks:

• Right-click on the Features toolbar and add Surfaces if it is not already shown
  
  
  
  
• Choose the type of surface you would like to create (extruded, revolved etc.)
• Create your desired surface
• Save the surface as a .STP file

In ANSYS:

• Create your project
• Right-click Geometry
• Select Import Geometry
  
  
• Locate your file and select it
• Go into Design Modeler and Generate

Now you should be able to see your surface in Design Modeler.