If you purchased recently from this site you'll no longer use a key.
You should have received an email with the link to download the full version.
If you didn't get that email try reentering your email address here:https://www.humblebundle.com/?s=resender
Once you receive the email, follow the link, and download the new setup file. Just install it over your existing installation (if you used the free version).
Please let me know if you're still having problems or still can't get the email.