The site wkstats is a 3rd party app, not anything WaniKani have made themselves.
You can get support for it in the thread [STATS] Statistics site
Essentially though, you should go to your API Key page, click Generate a New Token and then click Generate Token at the bottom of that page (without ticking any off the boxes) then copy the newly created code and go to wkstats.com, click Login at the top right and paste that code exactly as it appears into the box.
It should then give you your stats.