Grey screen comes up when try to edit a page in Flip program?

I start the Flip PDF Pro for Mac, but a grey screen comes up when I try to edit a page. The thumbnail icon has reappeared but I can't move the pages around to arrange the book. What can I do?

The problem is about Flash Player setting, please follow below steps to reset the setting to fix the issue:

1. Open the follow URL to set Flash Player security settings in Safari: http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager04.html

2.Go to the 3th tab to do "Global Security Settings":

3.Click "Edit locations...->Add location" in the "Always trust files in these locations" setting:

4.Browse for folder that Flip Application located which has been warned in "Adobe Flash Player Security" window, then click Confirm button:

5.Repoen the Flip PDF and you can continue process your file without problem now.