what kind of installation are you using? Gerben's i-installer or fink or have you installed tetex on your own? If you want to find out where different files reside, run this command:

kpsewhere mp-spec.mp

on my system (Gerben's architecture), I get:

/usr/local/teTeX/share/texmf.local/metapost/context/base/mp-spec.mp

/usr/local/teTeX/share/texmf.tetex/metapost/context/base/mp-spec.mp

The first file is the one actually used by your TeX-installation, so this is the one you'd need to modify. Actually, "all this unix-filename always repeating" is there for a very good reason, so don't complain about it...

Good luck

Thomas