mxd = arcpy.mapping.MapDocument('CURRENT')
df = arcpy.mapping.ListDataFrames(mxd, '*')[0]
newlayer = arcpy.mapping.Layer(theShape)
arcpy.mapping.AddLayer(df, newlayer, 'BOTTOM')
arcpy.RefreshActiveView()
arcpy.RefreshTOC()
del mxd, df, newlayer
How do I add a shapefile in ArcGIS via python scripting?