You propably tried to upload the whole zip and not the theme which is inside the zip you downloaed.
From the FAQ:
1. From your admin dashboard
When you are logged in as an administrator to your Dashboard go to Appearance -> Themes and select “Add New”.
From the dialog that pops out select your theme (for example: pokatheme.zip) and click to “Install Now”.
2. Upload your theme via FTP
You can use FTP client like FileZilla. Connect to your site and navigate to /wp-content/themes/.
Find the theme zip file inside your product zip and unzip it. (The zip you download from your profile contains several files except the theme zip).
The folder you unzipped (for example: “pokatheme”) upload it to /wp-content/themes/.
Lastly find the theme in your Dashboard Apperance -> Themes and activate it.