You've two choices during registration: Either via the internet (easiest) or manually.
You can do this by clicking the button indicated below on the Registration dialog that appears when starting the application.

Enter in the same email address and password that you first created above. The application will then check your account and find the relevant details and sort it out for you (meaning: you don't have to type in anything, yay!).
If you're not connected to the internet or don't want to use the internet method, click the other button. This is a more traditional "paste the code in here" style of registration.
For iShowU and Stomp, you need to enter the details exactly as they were emailed to you.
If I received this:

Then I'd need to be registering as follows:
