Hello ??
Just open the file adminlogindifferentuser.php and change this code
<form method="post" action="<?php bloginfo('wpurl'); ?>/wp-content/plugins/adminlogindifferentuser/wp-userlogin.php">
to
<form method="post" action="<?php bloginfo('wpurl'); ?>/wp-content/plugins/admin-login-as-different-user/wp-userlogin.php">
on line 28 and it should work now.