The Walk Designer uses a two-dimensional spline path at "ground level", along with prerecorded walk-cycle animations. It doesn't make allowances for altitude, surface collision, changes in terrain, etc.

You'll have to manually make keyframe adjustments to V3's yTrans channel to account for the stairs. ockham wrote a Python script that can drop a figure onto a horizontal surface, but you'd still need to tweak the animation to make it look natural. You'd best grow familiar with the animation palette and graph.

There are more powerful animation tools out there that can do exactly what you ask (NaturalMotion's endorphin, for one), but they aren't specifically made for Poser. I suppose it might be possible to import their animations into Poser as BVH files, though.