Download the zipped package from ThemeForest and extract to your desktop. In the extracted package you will find theme_name.zip file which is the WordPress theme.
You can install the theme in two different ways:
- WordPress: Navigate to Appearance -> Add New Themes -> Upload page. Select theme_name.zip file. Press the Install Now button to upload and install the theme;
- FTP: Extract theme_name.zip file and upload the extracted folder to /wp-content/themes/ folder on your server. Navigate to Appearance -> Themes and activate just uploadet theme.
For detailed WordPress installation instructions visit http://codex.wordpress.org/Installing_WordPress