Assuming "By registering PAN of your client in Income Tax site" means creating a login for the tax payer.
The First, Middle and Last name of the Assessee should be exactly the same as it was provided in the Form 49A which was submitted to the Income Tax Department while applying for PAN. If the first, middle and last names are not in the same order as in Form 49A, then the website will not allow you to register.
A simple solution to find out the first, middle and last name is to click on the "online tax payment" link in the Income Tax website, which would take you to your bank account's e-payment gateway.
There, fill all mandatory details and just enter a small amount and click "OK", the website will verify your name credentials by displaying the first, middle and last name of the Assessee before proceeding with the payment transaction. You can use the name as displayed.
Once you have got the name, abandon the payment and log out of the gateway.