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I can't install theme - Stylesheet is missing

If you have download the full package you need to first extract the file you've downloaded from ThemeForest, the theme is inside of it along with documentation, license, and other files. You need to upload only theme folder to your WordPress. This might be helpfull:

http://support.envato.com/index.php?/Knowledgebase/Article/View/269/0/my-wordpress-theme-isnt-working-what-should-i-do