Where you created the web form to put into your registration page, under "Web Forms" in the Admin area, you click on the registration form you created, maybe its called "Client Portal Registration" but I don't know what you called it. Inside that web form area near the top there should be four tabs: Edit Properties, Preview Web Form, Autoresponder E-mail, and More Actions. Click on Autoresponder Email. Check the box at the top, change what it says if you want to, and save it. I hope that helps!