2
the register page does not use a template, but utilizes the xoopsform classes. Its definition is stored in /include/registerform.php. To add your javascript modify its code like so:
$email_text = new XoopsFormText("", "email", 25, 60, $email);
Change to:
$email_text = new XoopsFormText("", "email", 25, 60, $email, 'onfocus="if(this.value=='Enter Email Address')this.value='';" onblur="if(this.value=='')this.value='Enter Email Address';"');