To add a WordPress Plugin using the built-in plugin installer:

  1. Go to Plugins > Add New.
  2. Under Upload, Choose a file.
  3. Find the WordPress Plugin you wish to install from your plugins folder within the theme folder you’ve previously downloaded.
  4. Choose one of the ZIP file.
  5. Click Install Now to continue with the installation. The resulting installation screen will list the installation as successful or note any problems during the install.
  6. If successful, click Activate Plugin to activate it.

Article taken from WordPress Codex: Managing Plugins