When installing an extension, you may encounter an error displaying a message like this:
Class 'Mage_Nameoftheextension_Helper_Data' not found in /.../.../.../app/Mage.php
That could happen due to several things, and the most popular are:
1. You forgot to copy files and folders from the "step_2" directory to your Magento root folder during installation. In this case the installation cannot be assumed as completed, you should copy files and folders from both "step_1" and "step_2" directories of the package.
2. You haven't cleared Magento cache right after the installation.
3. You haven't turned off the Magento compilation feature before the extension installation. In this case, we suggest rolling back current installation, turning the compilation off and making a new fresh installation of the extension.