I did some troubleshooting after this comment and did all that you said above. It appears to be a conflict with this theme and your plugin and what you both use as place holder for data. I'm not great with programming in WP, but I am familiar with core concepts so I'm going to try to explain it as best as I can. If you would like I can email you some screenshots.
The iFeature theme has two active widget sections: Footer and R sidebar. Even though there is no L Sidebar in the there, it's listed there in the widgets section on the top right. The box appears inactive at first glance, but apparently is a placeholder for certain styling in the theme. Specifically the backdrop (not the background but the backdrops for each picture and post) and some of the sizing scripts. If you go to my site the "backdrops" are the white boxes behind each post and image in the sidebar and in the feature boxes at top. When I install Displet Pop, the plugin uses this same placeholder location but overwrites it with the data for displet pop. The instant the plugin is activated the theme dumps the backdrops and related formatting and it whacks out the sight.
I hope that made sense, if not a couple screenshots I can email you would make it really easy to understand. The question here is this then. Is there another widget area that displet pop can be assigned to in place of the L Sidebar?
I'm currenlty using a different pop-up plugin that I hate and would really love to use yours.