Which version are you trying to download. 3.50 dollars (or 3.21 GBP) should unlock the 0.8 version of the game (the download option should have showed up when you donated). The 0.8 version of the game is quite old though. You might want to refund your purchase here and become a $3 dollar Patron on my Patreon page, which will give you access to the 0.14 version of the game.
And of course there is always F95Zone if you want the latest version for free without supporting me, but that's up to you.