I think you have downloaded only the WordPress theme file and not the whole package. Once you login to themeforest.net, you can see the downloads tab, click on the download button and you will see an option to download “All files & Documentation”. Please download it and extract the zip file. You will find the sample xml file, theme file, psds and Documentation inside the extracted folder. Go through the documentation thoroughly, where we have put lots of info with screenshots on how to install and configure your site using Extinct theme.
Besides, here is our youtube channel, where we have put some video tutorials for installing and configuring some of our themes. Most of the steps are very similar for all our themes