In order to use any of the VST plug-ins with Audacity, you must first install the VST Enabler.
Go to http://www.audacityteam.org/vst/ and download the install file for your operating system.
As with the plug-in installation, after you download and uncompress the file, you need to move it to the Plug-Ins folder (within the Audacity installation directory).
Restart Audacity, and the VST Enabler will be installed.
Remember, however, that you still need to download and install all VST plug-ins (after downloading and uncompressing them, you need to place them in Audacity's Plug-Ins folder, and then restart Audacity). You will then see these plug-ins in the Effect menu.