Here is an example on how to add the layer file to your current MXD:import arcpy
from arcpy import mapping
mxd = mapping.MapDocument("CURRENT")
df = mapping.ListDataFrames(mxd, "Layers")[0]
addLayer = arcpy.mapping.Layer(r"C:\DATA\PA_DEM.lyr")
mapping.AddLayer(df, addLayer)
arcpy.RefreshTOC()
arcpy.RefreshActiveView()
Here is an example on how to add the layer file to a particular group layer in your current MXD:import arcpy
from arcpy import mapping
mxd = mapping.MapDocument("CURRENT")
df = mapping.ListDataFrames(mxd, "Layers")[0]
targetGroupLayer = mapping.ListLayers(mxd, "L00 (147,748K)", df)[0]
addLayer = mapping.Layer(r"C:\DATA\PA_DEM.lyr")
mapping.AddLayerToGroup(df, targetGroupLayer, addLayer, "BOTTOM")
arcpy.RefreshTOC()
arcpy.RefreshActiveView()