This page will either email you your code, or, if you can provide enough information, allow you to download a donor.txt file directly. Note that if you get it wrong three times, you'll have to wait a full day before trying again. Too many failures and your IP may be banned. In addition, you're not allowed to download the file or send the email more than once per 24 hours.
To resend the registration email, you'll need to provide the email you used when paying, and the date when you paid. Leave the Merchant Account ID field blank. Remember to check your spam folders in case the email ends up there.
If you no longer have access to the email account you used when paying, you can download the registration file directly provided that you paid using PayPal. To download, fill in email, date and your PayPal Merchant Account ID. That ID can be found on the My Profile page when you've logged in to PayPal.