Private Sub WatchDirIsRunning(Sender ... {kommt darauf an wie die Signatur _
  deines Event aussieht} )
'// dein Code
End Sub
Dim wd As WatchDir {Deine Klasse}
Private Sub Form_Load (Sender As Object ....)
  AddHandler wd.{Name des Event}, AddressOf Me.WatchDirIsRunning
End Sub

Dim WithEvents wd As WatchDir {Deine Klasse}