Once you have download your theme from within your ThemeForest Downloads Page, you are ready install it within WordPress.
If you have chosen to download the installable .zip file, this is what you will be uploading to WordPress. Alternatively if you have downloaded the whole theme package, you will need to unzip it and get the installable WordPress .zip file out.
Note: Make sure that before installing and configuring your new theme that the latest version of WordPress is installed.
This is the easiest way to be able to upload your WordPress theme. Simply navigate to Appearance > Add New Themes > Upload. Then click the “browse” button and locate the zipped theme folder.
Click “Install Now” and your theme will be uploaded within a few seconds, so please be patient while it uploads.
Installing via FTP
Unzip your wordpress theme and upload it into the themes directory within WordPress. Here is a helpful article which will show you how to upload a Wordpres Theme manually via FTP: http://www.wpbeginner.com/beginners-guide/how-to-use-ftp-to-upload-files-to-wordpress-for-beginners/