It is relatively easy to build or run Rmpi under Mac OS X as long as the required component, LAM, is installed and configured properly. First get lam 7.1.3 for Mac OS X 10.4 from
http://www.lam-mpi.org/download/files/lam-7.1.3-1.dmg.gz
Edit /usr/local/lam-bhost.def to include nodes. If you mainly
run Rmpi under one machine such as Mac Pro with up to 8 CPUs, just add (if 8
CPUs are available)
localhost cpu=8
To install Rmpi from source, you may need additional tools such as c compiler. Otherwise just grap a universal binary (lam) version of Rmpi from Download.
CRAN now includes Leopard version of Rmpi. On a CRAN site, please go to "R Binaries" => "macosx"=>"leopard"=>"contrib" to download Rmpi_xxx.tgz.
Once downloaded, please use
sudo R CMD INSTALL Rmp_xxx.tgz
to install.