This error message is very common and means that WordPress cannot use the uploaded zip file as a theme. The issue is usually caused by uploading an incorrect zip file when trying to install the theme, like the complete package containing the theme, its demo files as well as many other stuff.
To fix this, just go back to http://themeforest.net/downloads and when clicking the Download button, choose the “Installable WordPress file only“.
If you wish for a more detailed explanation, here is an article from the Envato’s documentation that might also help you:
