Put the plugin in the Plug-Ins folder where Audacity is installed.
Then look at Help > About Audacity… to see which Audacity version you are using.
If you are using earlier than Audacity 2.1.1, simply restart Audacity.
If you are using 2.1.1, click Effect > Manage… then enable the plugin and click OK. See here for help:
Audacity Manual.
Please see Important information for Windows XP users.
Gale