You will need to edit the ~\CMSPages\GetFile.aspx file and add your Google analytics code to this file. You should receive JavaScript code with your account number from Google. Place this code into the GetFile.aspx file under the first line of code. Please see the example below:
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="GetFile.aspx.cs" Inherits="CMSPages_GetFile" %>
<script type="text/javascript">
var _gaq = _gaq || [];
_gaq.push(['_setAccount', '<YOUR ACCOUNT HERE>']);
_gaq.push(['_trackPageview']);
(function() {
var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
})();
</script>