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. Copy and Paste the nu.class file.
4. Click Start then type in %appdata%
5. Open up .minecraft, then the bin folder.
6. Right click on minecraft.jar, select open with... WinRAR Archiver.
7. Paste the nu.class file.
8. DELETE META-INF FOLDER.
9. Done, enjoy :)