There's instructions in the link, but basically you need to download WinRAR (http://www.rarlab.com/ ), go to 'Run' on the start menu (assuming you're on Windows) type %appdata%, find .minecraft>bin>minecraft.jar (right click>open with>WinRAR) and drop the mod files in where they're suppost to go, there should be a readme file in the zip file that the mod is in.
Here's the video for it http://www.youtube.com/watch?v=XZ8HtYglru8&feature=player_embedded
If you have any other questions just ask. Oh, it's worth noting that mods won't work on an update of Minecraft that it doesn't support, the Mo Creatures mod is on the current version, but when the next one comes out, you'll need to wait for any mods you use to be updated.
Jesse.
