You need at least PHP5.4 and Font-Awesome lib (css fonts)

FIRST:
If your MCE is messed up after installing this plugin, your server is running PHP under version 5.4!

Change the global php version in your server settings or try to add the following code to your .htacces file (wp root):

AddType x-mapp-php5.4 .php
AddHandler x-mapp-php5.4 .php

(the code can be different on your host, I am using 1und1)

Important: PHP5.5 resolves in compatibility issues with some older plugins or older WP versions. Try then 5.4!

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SECOND:
This plugin doesn’t include Font-Awesome, it just provides a button+short-code function to your MCE (works great!).

So you have to download the Font-Awesome pack and include it in your theme folder, then include the Font-Awesome css into your main-css file via @import or you copy the content of the FA-CSS to your main-css.

I think this plugin could be improved by a settings page with the option to include font-awesome or just some infos how to activate the frontend functionality of FA. (Not everyone is using a theme with FA included.)

Hope I could help you guys.