I was doing a "Santa Claus" Scene and I ran into a similar problem: My characters would not show any shadows on the ground. This is what I did: I took the Poser 2D plate and assigned a "glacier" map to it. Then I put it on the ground and scaled it up. This technique might work for you too. Another way might be that you use Bryce to make a flat terrain, assing an appropriate snow map and export the terrain as an obj.-file. This file can be imported into Poser. Make another terrain for "ice" and you're set.