In your application, choose "I only have a serial" (Also known as Product Key)
Example:
http://screencast.com/t/YcVkI9tZqsrc
After entering your credentials, and serial \ Product Key (in your email receipt), click Register.
If you have lost your receipt, you can get it from the order page:
https://store.stardock.com/myaccount/orders
Open the csv file and the serial(s) is in there.
Once activated, you will also be able to access your purchases from here:
https://store.stardock.com/myaccount/products