When did you buy it exactly? You should've received an email from Itch with a download link. You might've typo'd your email address if you never received an email. It would be good to get into contact with Itch.io's support in that case.
Oh, I'm sorry that happened! :(
Please get in contact with the Itch.io support, they'll be able to help resolve this issue for you: https://itch.io/support