Use the following shortcode [show_theme_switch_link] in templates to show the theme switch link.
Example: <?php echo do_shortcode('[show_theme_switch_link]'); ?>

You can also add Switch Mobile Theme link to your Menus from Custom Links section under Appearance > Menus.
Example:
Url : /?am_force_theme_layout=desktop (For Mobile Theme)
Url : /?am_force_theme_layout=mobile (For Desktop Theme)
Label : As you wish :)