I believe that you can recover the key through the Customer Menu available on the official website.
Access the following link to log-in: http://www.karaosoft.com/Cust_Login.aspx?topage=/CustomerMenu.aspx
If you don't have a username and password, then you'll have to contact the developers to pull the registration data from their records since they're the only ones who have this ability.
Contact page: http://www.karaosoft.com/TechSupport.aspx
Note: If you have a Facebook, account, then you can use their page to get in touch with them as well.