Today I encountered a problem with Magento. Every module I installed from the Magento Connect marketplace gave an error \”Fatal error: class \’Helper_Data\’ not found in domainfolder/app/mage.php on line 546\”. I tried a plugin, it didn’t work. I tried another, same error but different class name. After a while I knew what the problem was – the Compilation System was on and i had to recompile the whole code in order for the plugin to work. So if you have a error like I had, try the following steps:
- Go to admin Tools -> Compilation
- If Compiler Status Enabled, press the Run Compilation Process button
After this, the newly installed plugin should work. I hope this helps you.