First of all, you should make sure your index.php properly includes composer autoload.
<?php
include __DIR__ . '/vendor/autoload.php';
Then you need to validate your directory structure to be properly autoloaded.
If your PSR-0 configuration is
"psr-0": {
"DatabaseSeeder\\": "app/database/seeds/"
},
then your directory structure should looks like:
app/
database/
seeds/
DatabaseSeeder/
DatabaseSeeder.php
For more information about composer PSR-0 autoload, you can read composer schema documentation. Hope this helps.