i did that differently... u just edit the "email_activation" section in lang.lib.account_register.php under languages folder, to something like that:
"email_activation" =>
"Your account has been successfully created. An email has been sent to you with the activation link.</br> (If you don't receive your activation email, please check in your junk folder)",
i also edited the template and the css to show that message on top and bottom of the page and in a more visible way... ( after i noticed some users do not activate and try to login directly coz they didn't see that message...)
cheers.