Hi John,
a self registration is not possible. With a valid Certificate you should get an invitation if the Certificate was achieved after Oct. 1, 2013.
If done before Oct. 1, 2013 you should have received an e-mail from us.
List of valid Certificates can be found here: https://training1.sap.com/asset-manager/cdn/static-pdf/Valid_Cert_Exams_30_06_14.pdf/US/EN
I hope this helps.
Harald