Try 'locate mysql_config' on the terminal prompt, and see what that
gives you. (Or otherwise spotlight, but that may be avoiding unix
system dirs.)
Does the 'mysql' command itself exist? (Ie, can you enter that at the
command line?)
How was mysql installed on your system?
You set the the PATH to the executable, but you'll need to set the
PATH to the *directory* containg the executable. So
export PATH=${PATH}:/usr/local/mysql-5.0.51a-osx10.4-powerpc/bin/
(often, it's better to append the path than prepend it; otherwise you
may 'override' default executables.)
The 'mysql' may very well be located in that same directory, btw.
I would recommend that you set the PATH only for the one time you run
python setup.py build/install, so just execute the export command in
the terminal before running that (actually, once you've set the PATH
correctly, execute mysql_config first, to see things actually work.