How to Install Plug-ins In InDesign
Download the plug-in onto your computer and if necessary, decompress it (unstuff or unzip it). You should end up with a plug-in file, which usually has a file name extension of .pln or .framework. (Sometimes plug-ins on the Mac don't have a file name extension.)
To install the plug-in, locate the Adobe InDesign program on your computer and drag and drop the script file into the Adobe InDesign CS2 > Plug-ins folder. You can also put the plug-in inside a folder inside the plug-ins folder if you like (for example, David has a folder called "Third-Party Plugins" that he uses as an aid to organization.
Note that you may need to have an admin account on your computer in order to add plug-ins and scripts to InDesign - but once they're in there, they'll work for all user accounts.
Plug-ins only get activated upon launch, so if InDesign is currently running you should quit and relaunch it. Most plug-ins come with ReadMe files explaining how they work.