[Error On Campaign Zone Page]

I am getting an error when I access the Campaign Linked Zones page:

 

Fatal error: Call to undefined method OA_Central_Common::OA_Central_Common() in /usr/home/paul/TL6/AdServer/OpenX/lib/OA/Central/AdNetworks.php on line 49

 

I seem to be missing the PHP OA_Central_Common class. What is that and where do I get it?

 

Thanks,

Paul.