To add a WordPress Plugin using the built-in plugin installer:
- Go to Plugins > Add New.
- Under Upload, Choose a file.
- Find the WordPress Plugin you wish to install from your plugins folder within the theme folder you’ve previously downloaded.
- Choose one of the ZIP file.
- 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.
- If successful, click Activate Plugin to activate it.
Article taken from WordPress Codex: Managing Plugins