subscribe by email plugin - error failed to open stream

No special installation instructions were included in the zip, so assuming it is a standard install, I copied subscribe-by-email.php and subscriptions.php to mu-plugins. I then ran the incuded sql script to create the table.

Getting this error:

Warning: require_once(admin.php) [function.require-once]: failed to open stream: No such file or directory in C:\wordpress\wp-content\mu-plugins\subscriptions.php on line 28

Fatal error: require_once() [function.require]: Failed opening required 'admin.php' (include_path='.;C:\php5\pear') in C:\wordpress\wp-content\mu-plugins\subscriptions.php on line 28