Try the following:
Code:
import viz
viz.go()
r1 = viz.addRadioButton(0)
r2 = viz.addRadioButton(0)
r3 = viz.addRadioButton(0)
r1.setPosition([.5,.6,0])
r2.setPosition([.5,.5,0])
r3.setPosition([.5,.4,0])
#make sure r1 is not selected at start
r1.set(viz.UP)
vizact.onbuttondown(r1,viz.logNotice,'r1 selected')
vizact.onbuttondown(r2,viz.logNotice,'r2 selected')
vizact.onbuttondown(r3,viz.logNotice,'r3 selected')