I was having the same problem using this on my site http://www.smartdiversifiedincome.com
The popup was not popping up no matter what. I read on here that is was due to the theme and indeed it was however I didn't have to get a new theme.
here is what I did hopefully it works for you
go in to your header.php and locate this piece of code
<?php wp_head(); ?>
it should be located right before this code
If you do not see this code simply copy and paste it in.
If you do see this code delete it and place it where I just indicated so in the end it will look like this
`<?php wp_head(); ?>
no go to your footer.php
look and place this code
<?php wp_footer(); ?> right before this code
one thing to note is that in the footer I didn't have to erase the original
<?php wp_footer(); ?>code
so that code is actually in the footer twice (not sure if it matters, but the pop up now works perfectly.
Hope that helps.
Great plugin by the way