Try using 'DTrack' for the server name. Let us know if this helps.
Code:
TRACKER_MACHINE = "localhost"
VRPN_SOURCE = "DTrack@" + TRACKER_MACHINE
# VRPN Tracker connection (to ART Dtrack)
vrpn = viz.add('vrpn7.dle')
myHeadTracker = vrpn.addTracker(VRPN_SOURCE) #Default sensor is 0. Head tracker is on DTrack sensor 0
myFlystickTracker = vrpn.addTracker(VRPN_SOURCE,2) #Flystick tracker is on DTrack sensor 2
myFlystickButtons = vrpn.addButton(VRPN_SOURCE) #Flystick buttons
myFlystickJoystick = vrpn.addAnalog(VRPN_SOURCE) #Flystick analog thumb joystick
#Swap position and orientation to match Vizard's coordinate system
myHeadTracker.swapPos([1,3,2])
myHeadTracker.swapQuat([1,2,3,4])
# Also for the flystick tracker
myFlystickTracker.swapPos([1,3,2])
myFlystickTracker.swapQuat([1,2,3,4])