The first thing you need to do is to check whether the code arrived to other folders (eg. SPAM) or not. Access that folder in your email and locate the serial number. If it's not there, use the following email address:
firstname.lastname@example.org to get in touch with the developer.
If no method works, contact PayPal or your Bank (depending the way you used to purchase) and provide all the data to request a refund.