1. Open up the WinRAR Archive.
2. If you don't have an Archive extractor you can get one from here: http://www.rarlab.com/
3. Click Start then type in %appdata% or if on Windows XP 
click Run... then type in %appdata%
4. Open up .minecraft, then the bin folder.
5. Right click on minecraft.jar, select open with... WinRAR Archiver.
6. Drag and drop the nu.class and dj.class file into the minecraft.jar
7. DELETE META-INF FOLDER.
8. Done, enjoy :)
NOTE: If you encounter any errors, please post them on the forum and I'll be happy to help :D