If you want to make the directory available system wide, update the imports in your config file:
// autoloading model and component classes
'import'=>array(
...
'application.models.somethingfolder.*',
),
If you only use the class in specific places, you can import it before you first use it:
Yii::import('application.models.somethingfolder.SomethingModel');
// Or Yii::import('application.models.somethingfolder.*');
$somethingModel = new SomethingModel;