No worries! I think these are the steps you need to follow, although let me know if they don't work for you.
1. Select Download Now on the game's page:
2. Enter the minimum price for the paid version, which is $8 in this case, then select your payment method and complete your purchase:
3. You'll then be directed to the download key page, where you can download your purchased file(s).
I hope this helps!