I don’t know about the plugin you are using, but the way to add custom fields to the registration form and saving it to user profile is by hooking into register_form
action hook to output the field and then using the appropriate filter/action hook to update/save user profile.
Your case, for example:
<?php
// output the form field
add_action('register_form', 'ad_register_fields');
function ad_register_fields() {
?>
<p>
<label for="firstname"><?php _e('First Name') ?><br />
<input type="firstname" name="firstname" id="firstname" class="input" value="<?php echo esc_attr($_POST['firstname']); ?>" size="25" tabindex="20" />
</label>
</p>
<?php
}
// save new first name
add_filter('pre_user_first_name', 'ad_user_firstname');
function ad_user_firstname($firstname) {
if (isset($_POST['firstname'])) {
$firstname = $_POST['firstname'];
}
return $firstname;
}
?>