I discovered this problem because I was using a test site that has the default WP configurations.
The setting for permalinks can be found at: WP Admin > Settings > Permalinks and the Default structure was chosen which is:
Default = http://domain.com/?p=123
When I changed this setting to one of the following:
Post name = http://domain.com/sample-post/
Custom Structure = http://domain.com /%category%/%postname%/
Then the popup plugin worked fine.
What are you permalinks set to?