Thanks to Steven F. Koch for these helpful notes (steve.koch@up.ac.za)

Installing npRmpi on OSX 10.8 (Mtn Lion)

We basically need to install `Xcode' and its command line tools which
delivers compilers etc. then install MacPorts (www.macports.org) which
allows you to install R and openmpi, then set system wide environment
variables. Or you can install R directly from the R website if you
prefer.

1. Within App Store, install Xcode. It is free, but takes time to
install, depending on your connection.

2. Launch Xcode. Open Xcode Preferences. Under the Downloads tab,
install Command Line Tools.

3. It is now possible to install MacPorts. Follow the instructions
on http://www.macports.org/install.php. I downloaded the ".pkg"
version, and installed the same way as many other applications in Mac.

4. Install openmpi (and optionally R if you prefer to do this rather
than downloading the OS X binary from r-project.org) using MacPorts:
using Terminal (from Applications): type "sudo port install openmpi
R-framework" (without the quotes, and drop R-framework if you already
have R installed). For related information, take a look at
https://trac.macports.org/wiki/UsingMacPortsQuickStart

5. Next, we need to set system wide environment variables so that
these values will be recognized for both command line and GUI
applications such as RStudio.

Create or append these to your existing ~/.bashrc file from the
Terminal via `sudo emacs ~./bashrc' (without the quotes `') to set
environment variables for command line apps run from Terminal

export LD_LIBRARY_PATH=/opt/local/lib
export RMPI_LIB_PATH=/opt/local/lib
export RMPI_TYPE OPENMPI
export RMPI_INCLUDE=/opt/local/include/openmpi

launchctl setenv LD_LIBRARY_PATH $LD_LIBRARY_PATH
launchctl setenv RMPI_LIB_PATH $RMPI_LIB_PATH
launchctl setenv RMPI_TYPE $RMPI_TYPE
launchctl setenv RMPI_INCLUDE $RMPI_INCLUDE

If you use R-framework, you may wish to also set R_LIBS_USER to a
local directory. To do this create a directory as in `mkdir
~/Library/R' then also add the following to your ~/.bashrc

export R_LIBS_USER=$HOME/Library/R

and

launchctl setenv R_LIBS_USER $R_LIBS_USER

Then from the Terminal run `source ~/.bashrc' or log out and log in
again.

6. Test things out. From within R, type `install.packages("npRmpi")'
(without the single quotes) and if your install of openmpi and
R-framework went well and you set the environment variables listed
above, you can test out the install via `library(npRmpi)' (without the
single quotes) from within R.