AttributeError in Volume Control: 'gtk.HScale' object has no attribute 'add_mark'

see my message on https://bugs.launchpad.net/awn-extras/+bug/566567/

Traceback (most recent call last):
  File "/var/lib/python-support/python2.6/awn/extras/awnlib.py", line 1462, in init_start
    applet_class(applet)
  File "/usr/share/avant-window-navigator/applets/volume-control/volume-control.py", line 100, in __init__
    self.setup_main_dialog(prefs)
  File "/usr/share/avant-window-navigator/applets/volume-control/volume-control.py", line 121, in setup_main_dialog
    self.volume_scale.add_mark(100, gtk.POS_BOTTOM, "<small>%s</small>" % "100%")
AttributeError: 'gtk.HScale' object has no attribute 'add_mark'