You can get Snarlgribbly's EZMat, which does an emulation of his "Snow Machine" python script, here: http://www.runtimedna.com/EZMat.html Snarlgribbly's site is gone/taken down, but someone around here has re-hosted all his great scripts. You can find his separate "Snow Machine" python script, which does a wonderful job of creating realistic looking snow that has fallen on top of objects and the like, wherever that it. (Someone will probably followup with a link to those rehosted files.)

For terrain, Poser comes with some terrain that you could easily adapt to look like "snow", using textures and the morph tools. Get the Snow Machine script or use the EXMats script to emulate snow that has fallen on it and you're in business. :)

Do a quick search for "Free Morphing Terrain for Poser" and you should come across several freebies. There's one here that could be adapted: http://www.planit3d.com/source/poser/p_props.html