I am not sure you are attending certification in which country.
In India, I am not yet sure if you can still directly go and register with Pearson VUE.
If your are in other eligible countries, you need to contact SAP Education, they would send you a form with your Debit / Credit card details, then they would provide you a S ID. They would also provide you a voucher code, which can be used on the Pearson Site to register. This Voucher is normally valid for one year from the date of issue.
Hope this helps you.