The XLRstats plugin is included in the B3 (BigBrotherBot) distribution package. You need at least B3 version 1.10+ to be able to use all features that the XLRstats website v3. brings. It will still work in a limited way with older B3 installs, but you'll miss a lot of nice additions.
Website / Webfront v3.
The installation instructions for the website are included in the package, you can read the latest instructions on our GitHub master branch here.
Website / Webfront v2.
This older version of the website is no longer supported. But it is still available at GitHub. The installation instructions are in the readme.txt file in the v2. package.