This plugin allows the user to create single-id.php pages and customize them. The id corresponds to the Category ID that is being queried.
Double check that your single-id.php is actually refering to a cat id. For example if your cat id is 5, then single-5.php should do the job. Also make sure the file resides on the theme's root directory, that is alongisde its fellow single.php.
If that doesn't help, try enabling WP_DEBUG on your wp-config.php and read the errors. If any problems show up, either post them on the support forums and I'll do my best to help, or see what you can do to solve it.