You need to log into gmail. The builder uses your gmail account to authenticate you. If there is a conflict between the email address you bought with and your gmail account, contact iRule support and they will sync everything up for you (This happened to me and was fixed VERY quickly).
From the buy now page at iruleathome:
Warning: the iRule Builder uses a Google account (Gmail) to log in. Make sure that you complete your purchase of the iRule Builder license using your Gmail email account. Your Gmail account will be your username and password to log into the iRule Builder.