Just done this on mine to check.
1) Open the Browser ( I just used the Silk Browser which comes with the Fire Tablet but would work with Google Chrome)
2) Go to wildcamping website. Make sure you are logged in.
3) Using the links on the top click on location app.
It opens in the Browser - you don't install an app in the normal sense.
It runs in the Browser. You can find instructions by clicking on the settings symbol ( the cog wheel thingy at the extreme top right.