Does it matter which version of the tracking code I am using?
Yes. Most people use the Traditional or the Asynchronous Syntax tracking code (ga.js). This plugin works for these tracking codes. Recently Google introduced a new tracking code (analytics.js) which is currently in public beta fase. This plugin doesn’t work for this tracking code yet. As soon as more people are going to use the new tracking code, I will put a setting in the (future) option page where you can define your tracking code version.
Google Tag Manager is new to me. In the future I will definitely look into it, because it looks quite interesting. Still lack of time at the moment…