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. Note that these instructions also apply to importing any part or assembly from Solidworks to ANSYS; you just need to skip the step  in Solidworks on adding Surfaces.

Method I

In Solidworks:

 

In ANSYS:

 

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

 

Method II

After you have created the geometry in Solidworks and saved it:

In ANSYS

Now you can edit your Geometry in Solidworks and refresh it in Design Modeler after you have changed it.

 

To refresh the geometry in Design Modeler:

Now your Geometry in Design Modeler will reflect the changes you made in Solidworks. 

 

Method III

 

In Solidworks you can connect the geometry to Workbench:

ANSYS Workbench will then open with the Solidworks file attached: 

 

 

Now you should be able to update your geometry in Solidworks and refresh it in ANSYS