1) Do you want labels above or below the slider? If so, you can create text objects and position them at those intervals.
2)Register a callback function for the slider event using
vizact.onslider:
Code:
import viz
viz.go()
slider = viz.addSlider(pos=[0.5,0.5,0])
def sliderChanged( pos ):
print pos
vizact.onslider( slider, sliderChanged )