In order to preview documents, the files should have the sharing permission "Anyone with link can view". The plugin will automatically change the sharing permission if you have set the following on the Advanced tab of the main plugin options page:
|Your Google Apps Domain
However, in some case the plugin isn't able to manage the sharing permissions even with those setting enabled. In that case, please try to set the sharing permissions manually via the native Google Drive interface.
If that also doesn't help, something else is going on. In that case, please create a support ticket and provide your purchase code and a temporarily WP admin login so we can safely debug the issue for you.