It would be helpful to state what you were doing when you got this error.
I am going to "assume" that you were installing the Active Users v4.0 MOD, am I correct? Did you follow every single step in the readme.html file? Sounds like the code that was to be added to inc_functions.asp, didn't get added.
I must admit that I feel very dumb now. I apologize for wasting your time. It seems as though I simply forgot to upload the new config.asp file. Sorry. Thanks for the help though.