Hello,
Do you have access to your httpd error_log file? Does it provide any additional information? It sounds to me like an include is failing, and as a result, a function is not avaliable. You might be able to find out exactly which file isn't working by checking the httpd log (at least for apache, this is generally where php errors are logged)
In the meantime, you did change only the paths that were specified in the install intructions right? That error dones't really look like you are missing a file path setting, but it could be caused if you changed a path that you wern't supposed to.
Exuardon~