Getting the scripts
Download the latest version of the zootoolbox from http://www.macaronikazoo.com/ where updates and bug fixes are posted every couple of months. The RAR file contains mel scripts, text files, icon bitmaps and an example rig.
Everything has a place
When you installed maya the installer put everything in somewhere like C:\Program Files\Autodesk\Maya2008. This should be treated like a system folder. Anything you do to customize maya - like installing new mel scripts or plugins - should be done in local folders that are in a seperate location to the maya install. That way if you have a problem it is much easier to fix.
The default location for maya user files is something like C:\Documents and Settings\david\My Documents\maya\2008 but you can specify another location if you wish. This can be done by creating an environment variable - right-click "My Computer" and choose "System Properties" and goto the "Advanced" tab and click "Environment Variables", then add a new variable to the "User variables" section as shown here
The image above shows an environment variable called "MAYA_APP_DIR" which I have set to u:\david\maya. When you load maya it creates your user preference folders starting with u:\david\maya\2008. This is where you can customize the way maya works. This is where your custom scripts will be located - in a folder called u:\david\maya\2008\scripts. You icons will go in u:\david\maya\2008\prefs\icons. (Unless you are called david, you may want to change the MAYA_APP_DIR to suite your needs - just don't put it inside the maya installation folder).
It is amazing how quickly you can end up with a huge collection of mel scripts. If you dont keep them organized you will end up in a mess. There are about 125 mel scripts in the zootoolbox as well as some bitmaps (mostly in the xpm format). I suggest that you give the zoo mel scripts their own folder within your main scripts folder - for example U:\david\maya\2008\scripts\zoo. This will keep them from getting mixed up with other custom mel scripts that you might download, and will make it much easier to update in the future.
Copy all the files from the scripts folder in zooToolbox.rar to U:\david\maya\2008\scripts\zoo. Then copy all the files from the prefs\icons folder in zooToolbox.rar to U:\david\maya\2008\prefs\icons. You would have also noticed some files called dagMenuProc_v2008.mel (and similar) in the top level of zooToolbox.rar. Copy the one whose name matches your version of maya into U:\david\maya\2008\scripts\zoo and then rename the file to be dagMenuProc.mel (more on this later).
(Currently the 2008 version of dagMenuProc.mel is not in the RAR, but you can download it from here)
So you now have the scripts where you want them, but we need to tell maya where they are. By default maya will find any scripts that you place in U:\david\maya\2008\scripts but it will not look in the zoo folder unless you tell it to. The best way to do that is to use the Maya.env file which is located in U:\david\maya\2008. If you dont have this file you can simply make a new text file called Maya.env and open it in a text editor. This file is like the system properties environment except that it only applies to maya and is much easier to work with.
To specify locations for maya to find mel scripts, add the following, all on one line, to Maya.env
MAYA_SCRIPT_PATH = u:\david\maya\2008\scripts;u:\david\maya\2008\scripts\zoo;
These are file paths seperated by a ;. When you restart maya it will be aware of any mel scripts contained in either of those locations.
It is important to know that any scripts found in your customized MAYA_SCRIPT_PATH will take precedence over any script with the same name in the maya installation. The script called dagMenuProc.mel is a modified version of one of the original maya scripts. The zoo version of this script has a few extra lines to add some useful zoo functions to the maya right-mouse-button menus.