Extract the package downloaded from ThemeForest to your desktop. In the extracted package you will find"theme_name.zip" which is the WordPress theme file.
You can install the theme one of two ways:
Extract the theme_name.zip file and upload the extracted folder to the /wp-content/themes/ folder on your server.
Navigate to Appearance > Add New Themes > Upload page. Select the theme_name.zip file. Press the Install Nowbutton to upload and install the theme.