I have no clue how to download my kickstarter reward from Itch.io. This is the les userfriendly site I've ever visited.
If you've claimed your key already, there should be a download option at the top of https://rocketjumptechnology.itch.io/king-under-the-mountain
Your key should have been emailed to you by BackerKit, but if you've missed that you can retrieve it at https://kingunderthemountain.backerkit.com/