You should have received an email from firstname.lastname@example.org titled "Welcome to PitchMagic" with your order confirmation details. In that email is a link to "Create Your Account" under the heading "I can't sign into PitchMagic". Click that and you'll be able to create your account.
Alternatively, you can go to https://www.pitchmagic.com/sign_in.php#trouble and click the link titled "Did you just sign up? Make sure to create your account."