Check whether compiler mode is on, if yes, recompile to have the MageSetup classes in the compile_dir

[Schrank]

Compiler mode copies all classes to includes/src/ and changes the INCLUDE_PATH. If you ask me, compiler mode is bullshit, but people use it. After installing MageSetup via MagentoConnect, the modules are not recompiled, therefore magento will not find the classes and throw an error:

PHP Fatal error:  Class 'FireGento_MageSetup_Helper_Checkout_Data' not found in /var/www/app/Mage.php