The text on the site has not yet been updated, but the download file is actually 3.3.
Make sure you are downloading the right file. This is the link:
https://prolific.s3.amazonaws.com/Ma...eAquarium3.exe
Let me know if pasting in your Key Code doesn't work with this. (Also make sure you're not accidenatlly grabbing spaces or extra characters when you copy and paste.)