This is an example macro from API help (Fire event after selection)

I am not able to run this

Is there anything wrong with this code

Someone please help

'------------------------------------
' Preconditions:
' 1. Part, assembly, or drawing document
'    is active.
' 2. Run this macro (press F5 and click Run).
'
' Postconditions:
' 1. Select an entity.
' 2. Message box is displayed confirming your
'    selection.
'----------------------------------------
Option Explicit
Dim swApp As SldWorks.SldWorks
Dim swModel As SldWorks.ModelDoc2
Dim partDoc As New Class1
Dim assemblyDoc As New Class2
Dim drawingDoc As New Class3
Sub main()
Set swApp = Application.SldWorks
Set swModel = swApp.ActiveDoc
' Determine the document type, then
' execute its corresponding class module
If swModel.GetType = swDocPART Then
    partDoc.init swModel
ElseIf swModel.GetType = swDocASSEMBLY Then
    assemblyDoc.init swModel
ElseIf swModel.GetType = swDocDRAWING Then
    drawingDoc.init swModel
End If
End Sub
'Class1 module
Public WithEvents doc As partDoc
Public Function init(ByRef docin As Object)
    Set doc = docin
End Function
Private Function doc_UserSelectionPostNotify() As Long
    MsgBox "Entity selected in a part document."
End Function
'Class2 module
Public WithEvents doc As assemblyDoc
Public Function init(ByRef docin As Object)
    Set doc = docin
End Function
Private Function doc_UserSelectionPostNotify() As Long
    MsgBox "Entity selected in an assembly document."
End Function
'Class3 module
Public WithEvents doc As drawingDoc
Public Function init(ByRef docin As Object)
    Set doc = docin
End Function
Private Function doc_UserSelectionPostNotify() As Long
    MsgBox "Entity selected in a drawing document."
End Function