Less than a week ago, I've paid with PayPal for YTD Downloader to switch to YTD Pro. Why do I still have the basic version? Why didn't my app changed?
From the moment you have paid for the application, an email should have been dispatched to you with a serial number. You need to look on your email for message. Also check in the SPAM folder to see if the email got there.
If there is no email, then access the following page and report this issue to the developer: http://www.ytddownloader.com/contact_us.html#