I just tested the plugin with your page and it seems to be working fine for me, so the error is probably in your Facebook application.
Could you go to your application on Facebook and check your
Advanced Settings? Here is a screenshot of what a working set-up could look like (all defaults).
Especially the *Require AppSecret Proof for Server API calls: Disabled* and *App Type: Web* are important.
Also, try switching to the development version of the plugin as it includes a button to test your configuration and some other improvements. Just overwrite your existing plugin files with the files from the zip-file.
Hope that helps!