Worldpay extension for Magento

I have a Magento store setup and installed this extension:

http://www.magentocommerce.com/magento-connect/worldpay-extension.html

when you goto:

Admin>Configuration>Payment Methods

you are greeted with this php error message:

Fatal error: Class 'Phoenix_Worldpay_Helper_Data' not found in < my website >/app/Mage.php on line 516.

From what I have read about helper classes it is looking for the directory the helper class is in.

The helper class exists and is in the right location:

/app/code/community/Phoenix/Worldpay/Helper/Data.php

Im using Magento community v 1.6.

Anyone have any suggestions?

The possible causes for this I am aware of are:

• File does not exist. (You've checked this)
• File exists but has wrong permissions, typically it should be 644 (read-write, read, read)
• File exists but class name is wrong. It must be the same as the path with / replaced by _
• File exists but code is malformed so class is never registered.
• Compilation is enabled and hasn't been rebuilt.