The file you get from Themeforest is a zip file that you’ll need to unzip on your computer. Once unzipped you should see a couple of folders and files. One of those is labeled choices.zip (or just choices if you aren’t showing extensions).
It is this file that you can upload via the WordPress install themes tab where you select a zip file to upload.
If you are installing via FTP, unzip the choices.zip folder and within you should see choices and _MACOSX. You want to install just the choices folder into your themes folder on your server’s wordpress files.