To import the files right-click inside the grey box on the left of the timeline controls, and click add media to project. Once your media is in the project drag all the files to the timeline.
Make sure to only import image avi and wav files.
Step 1: Chroma-Keying or "Greenscreening"
Click on the Video Plugins Tab on the bar near your media library, then click Chromakey, and drag it onto your movieclip (A movieclip can be identified by a gray bar on the time line).
Then expand the ChromaKay drop menu and change the color to what you wish, also this will not work without sliding the tolerance bar to the right setting for you.
Note lighting is very important and will decide whether or not your ChromaKey works.
Step 2: Particles "Making It Snow"
Go back to the video plugins tab on drag Particles onto your video clip. First if making snow uncheck the "Blend" box and check the "Linear Color" box. Then in the Creation dropdown menu change all colors to white and change the size to 1-5 (min-max). Then change the colors under the destruction menu to white also. Next move the timeline marker to the middle of your video, and open the Emitter dropdown menu and change the settings Of the radius to fit the whole video, and the dir of the "Y" to a negative number so the snow flows down, from then all the settings are fine to edit just make it how you want it!