cgi script for changing error page

I have a client who has been trying to change her 404 error page with a
cgi-script I found at http://cgi.resourceindex.com .  Today she sent me this
message:

> well, i've been messing with this cgi thing for hours to no avail. i
> couldn't use that script file you pointed me to because it was a zip
> thing and well... apple, you know? i did find another source on that
> site and tried that but i have a few things that stump me.
>
> it says i have to enter a command "chmod 755". where do i enter
> that?? how do you enter unix commands? let's make that my first
> problem to solve. and where is the error log file? i looked through
> everything i could get my hands on in and around my root folder and
> still could not find anything about errors.
>
She's working on a Mac and I really don't know how to set up this script
either.  Can anyone help out?
TIA!
Blue

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