I've just purchased two themes from Themeforest, and can't install either of them, for different reasons:
1) The first one says 'The package could not be installed. The theme is missing the style.css stylesheet.'
2) The second just presents a near blank screen saying 'Are you sure you want to do this?', the tab is entitled 'Wordpress failure notice'. I suspect this has something to do with the file size maybe, which is quite large.