After download the full package from theme-forest, you will get a zip file inside your main package. You can install it from word-press back-end or manually.
install from wordpress back-end :
1. Go to wordpress Dashboard->plugins->add new. 2. From here click upload plugins button, then you will find the plugin upload window. 3. Upload the zip file then click install now button. 4. After successfully install the plugin then active it.