Issue on the installation

You have to delete the plug-in in your wp-contents/plugin folder.
I had the same problem.
This plugin does not work with my WP version : 3.8

This plugin with version 3.8 worked for me.

Did you try to reinstall it?

oh, I’m confused.
This website works with 3.5.2 version, not 3.8

Does it work with 3.9 for you?

Yes, I’ve just tried with 3.9 and it’s ok.
(I’ve tried the widget)

Then lucasromao, try it on WordPress 3.9.

What about version 3.9.2? I have tested it with this version and it works great.