Microsoft.Office.Core.MsoTextOrientation.msoTextOrientationHorizontal
is located in OFFICE .COM NOT OFFICE.NET
THIS WORKS
rng = oDataDoc.Tables[1].Cell(1, 1).Range;
xShape = oDataDoc.Shapes.AddTextbox(Microsoft.Office.Core.MsoTextOrientation.msoTextOrientationHorizontal, 1, 1, 200, 200, ref rng);