There are several ways of doing it. 

If you want total whiteout on your leaves, jjust set the overall colour to white in the colour correction box on teh colours and alpha tab

If you want only bits of snow, use a mixed material driven by a fractal and you can also set the height and slope functions on the influence and environment tab.

If you desparately need something, IM me with your email address and I'll send you one of mine.

Gill