You must assign a
MsoShapeStyleIndex to the
ShapeStyle.
To get a rectangle with No fill and black border Try this
WordDoc.Shapes.AddShape(1, 30, 15, 565, 760).ShapeStyle = Microsoft.Office.Core.MsoShapeStyleIndex.msoLineStylePreset1;
The enum
MsoShapeStyleIndex has many more options, try them to get your desired shape.