Disabling Parent Window While Pop Up Is Open

I used the following blog to display an add page after a button in a tab is clicked. I was able to get my popup window to work but can't figure out how to disable the parent window and grey it out until the popup is closed. http://xlinesoft.com/blog/2013/04/25/displaying-a-page-in-a-popup-window/, the parent page is still active and the users are able to click
Can anyone help??