Click on start, and in the search bar, type %appdata%, then press enter. Click on .minecraft, then on 'bin'. Open the minecraft.jar file with a program like WinRar or 7Zip. Click on the META-INF folder inside minecraft.jar, and press the delete key. Confirm the deletion (the mod won't work unless this is deleted).
Open your downloaded MenuImprovements .zip with WinRar or 7zip and drag all files into the minecraft.jar file. Close the WinRar/7zip windows, and open Minecraft. If it doesn't work right, just leave a post below.
For Mac and Linux users, it's pretty much the same process. Just open the .zip, and drag everything into your minecraft.jar which is located at /home/username/.minecraft/bin/minecraft.jar on Linux, and at /home/username/Application\ Support/Library/minecraft/bin/minecraft.jar on Mac.
Hope you enjoy the mod!
Here are some screenshots of the mod (more soon):
Jan 11, 2012: 1.0 - First release
Jan 11, 2012: 1.1 - Removes "Mods" from "Texture Packs" on the main GUI.
Jan 27, 2012: 1.2 - Adds support for Minecraft 1.1 and re-organizes the ingame menu.
Jan 28, 2012: 1.2.1 - Fixes a bug that hid the Options button in the in-game menu
Jan 29, 2012: 1.2.2 - Adds two new features (screenshot button, and change world)
Feb 1, 2012: 1.2.3 - Adds a new feature - Minecraft Wiki button that takes you right to the wiki
It would be nice to know exactly what buttons are added before downloading, so I can see if I could use it or not. And add a picture so you don't get blasted with the "no pics no clicks" crap. I do not think they are necessary, just what buttons are added.