Just a note, the minimum required version listed for this plugin is 2.7 but it uses a call only available in 2.8 - this means if you have a 2.7-based blog (WHICH YOU SHOULD NOT STILL HAVE! Upgrade now!) then in certain circumstances the tracking code won't work properly. I'm not sure exactly what the circumstances are - haven't looked.
The line I noticed is this:
return plugins_url('', __FILE__).'/';
I'm pretty sure that using plugins_url() with the __FILE__ argument didn't work until 2.8 came around. For any <2.8 blogs it'll leave you a lot of 404 errors for a file called:
because plugins_url() will return the wrong value.