There's no straight forward link or action list of how to register a Sun's Certification Exam, here is what I got after hours' search:
1. Enter Sun's website to purchase the exam you want to take, use the following link:
http://suned.sun.com/US/certification/java/java_progj2se.html
Select the exam you would like to take, then add it to the cart and proceed checkout.
You will get the Exam Voucher Number when your purchase is completed.
2. Call Prometric (800) 795-3926 to make an appointment for your desired date and location.
Use the Exam Voucher Number you got above.
You will go crazy if you insist on registering online, cause Prometric's website is sooooo confusing and misleading!