[Jeff]

You need to assign a value to pos before you add to it:

import viz
import vizshape
import vizact
viz.go()

cube = vizshape.addCube()
cube.setPosition([0,1.5,3])

pos = 3

def moveForward():
    global pos
    object = viz.pick()
    if object.valid():
        pos += 0.5
        object.setPosition([0,1.5,pos])
        print 'move forward'

vizact.onkeydown( 'w', moveForward )

def moveBackward ():
    global pos
    object = viz.pick()
    if object.valid():
        pos -= 0.5
        object.setPosition([0,1.5,pos])
    print 'move backward'
    
vizact.onkeydown( 's', moveBackward )