Frank Klemm has put up intructions regarding the use of MPC replaygain:
http://www.uni-jena.de/~pfk/mpp/#replaygain
Also new decoder & replaygain version is available here:
http://www.uni-jena.de/~pfk/mpp/bin/mppdec-windows-0.98w.zip
Winamp plugin in this package is for some reason named winamp.dll. It must be renamed to in_mpp.dll and copied into WinAmp/Plugins directory.