Both the server and the add-on suffer from serious bitrot. Since Firefox 32 is released it is known that some users can not connect anymore.
Rewriting the original authors add-on and server in Node.js is somewhere on the back of my mind but I don't see it happen anytime soon, because of a lack of time. If you are interested in leading or helping with this effort please let me know.
You can manually add this proxy to your add-on settings using the following data:
Note: if you experience 413 or 400 Bad Request errors please use the customized GoogleSharing add-on.