Nope it's not registered, which is what I thought. But is it really possible to buy AppleCare without giving any identifiable information as to what it protects?
An AppleCare Protection Plan for a MacBook (or iPod or iPhone or Mac Pro tower or display) is a stand alone product. It is sold in many places just like any box of software. It's up to you to open the box and enroll the product using the serial number. When you purchase an APP you do not need to provide a serial number. The purchase is not the transaction that enrolls the plan for your product. If you never used the enrollment number in your box to apply to the serial number of your product over the phone (or online or by mailing in the enrollment card), then your product can't be covered.
If you buy your computer at an Apple retail store and purchase an APP at the same time then they will ask you for your contact information when you pay and they can enroll it for you right then and there. But, if the product and the APP are purchased separately, they're not tied together until you go through the enrollment step.
If you have a receipt for the Mac and a receipt for the purchase of the AppleCare Protection Plan, you can call AppleCare Support (in the US at 1-800-APL-CARE), there will be an option to talk to someone about AppleCare itself rather than "technical support". A person there can probably contact an AppleCare specialist you can fax your receipts and the enrollment number and they MAY be able to enroll your computer. If you don't have both receipts and the enrollment number, they won't be able to help you.
If you really don't have the original AppleCare Protection Plan information, I'd suggest you buy a second copy and enroll it with the new number. If you find your original plan's information later, you can resell it to someone else who can use the enrollment number to apply to their Mac's serial number.
-Doug