My fault. Bend is not a valid code. YTRAN, ZTRAN and XTRAN are what I need to use. So the following code works -

import poser
import random
scene=poser.Scene()
scene.SetFrame(23)

MyFig = scene.Figure('Terri')
ChangeParm = MyFig.Actor("Left Shin").ParameterByCode(poser.kParmCodeYTRAN)
ChangeParm.SetValue(5)

I'm used to working with Bend values, so I just have to figure out appropriate YTRAN values instead. The next step is to add some randomness to the parameter changes to make movements look more natural.