Having CSS/JS issues with the PopUp! plugin backend

I manually installed the PopUp! plugin on my WordPress blog, but the backend doesn't seem to load all CSS/JS files properly. Please see the attached screenshot to see what I mean. You can see that the filters aren't loaded properly and can't be set via drag & drop. Basically any configuration except for adding content isn't quite working.

The WordPress is 3.9, the plugin is latest version (downloaded today).

Hope you can help me and thank you for taking the time to read this!