Alter Registration Form Text
you need to call the function YOURTHEME_form_alter(), where YOURTHEME  is the name of your theme. Eg, the Bartik theme would have such a function called bartik_form_alter() ... you must also clear your drupal cache for the changes to take affect.

EDIT

you make your code like this:

function YOURTHEMENAME_form_alter(&amp;$form, &amp;$form_state, $form_id){
    switch($form_id) {
        case 'user_register_form':
            $form['account']['mail']['#description'] = t('E-mail Address');
    }
}


if you call your theme "sky" it would be sky_form_alter.