Following the link from step 1 will get you to your account page. Also below is a video that details the registration procedure.
To register your copy of Sequence Generator Pro:
1 - Create or Login to your Main Sequence Software Account: http://www.mainsequencesoftware.com/AccountManagement
2 - Enter the license key(s) above into the “Redeem License” text box and click “Submit”
3 - Download and install the latest version of Sequence Generator Pro
4 - In Sequence Generator Pro, click on the Help menu and select “Register this Machine”
5 - Enter your account information from step 1 (email and password)
6 - Restart Sequence Generator Pro
That’s all!