You need to install TamperMonkey. It is a browser extension.
Then head over to The New And Improved List Of API and Third Party Apps - WaniKani / API And Third-Party Apps - WaniKani Community. Follow the links to get the WaniKani Open Framework. You HAVE to install this first (or change the order in TamperMonkey- just easier to install it first).
From there, the script you want is [Userscript] Wanikani Heatmap - WaniKani / API And Third-Party Apps - WaniKani Community.
As long as you install them correctly, you should see that heatmap on your dashboard. Hope that helps!