No problem. Don’t be dazzled by all the text. You probably only need the FTP part and you can check the other bits for learning and reference .
And if you fail, try again. There always is a first time. Goodluck.
This can be two things really.
1. First make sure you are installing the correct ZIP (installable Wordpress ZIIP and not the all files and documentation ZIP)
2. A few multipurpose themes are too big to install through the WordPress admin. You can upload these by using FTP instead.
Perhaps you bought an HTML template. Check the category of the file you bought. HTML files can not be used in WordPress.
If the item is not in your downloads list that probably means that the purchase is done with a different account. If you happened to contact the author in the past there is a possibility to trace the account details and sign in with that account.
1. Live demo and /or screenshots 2. Item comments 3. Portfolio of the author 4. Update of items (when applicable) 5. Reputation of the author. 6. Slightly related: Behavior in item comments and /or forum.
This is a general forum. Theme specific help can be found here.
Just a few general pointers. Make sure you followed all steps as described in the theme documentation. This documentation can be obtained by going to your download section and choose download all files and documentation from the download button. If problems remain after that contact the author through their provided support channel. Goodluck.