[metapost] Where is mfplain.mem supposed to be?

[Karl Berry]

    I need mfplain.mem (which, apparently, is missing from TeX Live), 

Happily, it's already been enabled for next time.

    into the same location as mpost.mem, 

That should be ok.

    but then mpost does not find it:
    mpost '&mfplain' foo.mf
    Sorry, I can't find that mem file; will try PLAIN.
    I can't find the PLAIN mem file!

    Should I put mfplain.mem somewhere else? Or should I update some other 
    file in the distribution?

Did you run mktexlsr?

karl