Try this. Open plog-admin/plog-includes/install-form-setup.php and find the code below on line 30: <form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post"> and change it to this: <form action="" method="post">
That should work for you. However, it is odd that your server does not have PHP_SELF defined in it's server superglobal. You may want to check with your webhost to see if they can fix it from their end.