Having your PSR-0 namespace
"Acme": "app/"
Your class file must be:
/var/www/appdir/app/Acme/Services/TaskCreatorService.php
And you have to
composer dump-autoload
Once, when you created the PSR-0 namespace. Check the file
/var/www/appdir/vendor/composer/autoload_psr0.php
And check if your namespace is there.