Was trying to install the MeanBee Royal Mail Domestic and International Shipping Extension via Magento Connect. It installed fine, but when going to System -> Delivery Methods - I got the error :
Fatal error: Class 'Meanbee_Royalmail_Helper_Data' not found in Mage.php
I have this problem sometimes with extensions and the problem is simply that I have system>tools>compilation \’Enable\’, I disable it then configuration works I set it all up them re-comile!
Just go to System -> Compilation and Disable it! fixed.
Original link: here