I purchased Things 3 for my MacBook Pro but it won't download

I purchased Things 3 for my mac but it won't download. I have rebooted my computer. I also got a refund last week and did a trial, hoping it would automatically stay on my computer when I paid for it, but it deleted it when I paid today and I am still getting the awful spinning circle. Any ideas to help me?

[Re-Titled by Moderator]