All you need to do, is download the texturepack file and place it (still in it's zip file) into your "texturepacks" folder in your minecraft folder. (By default (in win7) it's username/appdata/roaming/.minecraft/texturepacks)
Then when Minecraft is loaded, there is a "texture packs" button on the front page, click that and it will show all the texture packs in that folder. Just select the one you want to try, wait a few seconds for it to load - then get into the game and see how it looks.
