I added that to the bottom of my headinclude template and works for me.Hope you get it sorted.
I found the JS file online, and uploaded it to the OP's server, and added the script via a plugin, and it works right out of the box with no additional HTML needed.
Inked_Mono, if you don't like this particular effect, simply delete the plugin titled "Add Snowstorm" and delete the file "clientscript/snowstorm.js" from your server.