Connecting R to Postgres on OS X is a little bit of a hassle given where OS X looks for the libraries to link to.

Assuming you've installed Postgres via MacPorts, add shell variables for your Postgres lib and include directories to ~/.profile

export PG_LIB_DIR=/opt/local/lib/postgresql82
export PG_INCLUDE_DIR=/opt/local/include/postgresql82

Add a link from the OS X SDK directory to /opt

$ sudo ln -s /opt/ /Developer/SDKs/MacOSX10.4u.sdk/opt

Then install RdbiPgSQL as you would normally.

If the install fails because R isn't picking up the PG_LIB_DIR, _INCLUDE_DIR variables, keep a lookout for the lines in the install error output that look like

The downloaded packages are in
        /tmp/RtmpTakYpZ/downloaded_packages

In Terminal, change to this directory, replacing the 'RtmpTakYpZ' string with whatever directory the error output gives. Then run the R install command on the package file:

$ cd /tmp/RtmpTakYpZ/downloaded_packages
$ sudo R CMD INSTALL RdbiPgSQL_1.10.0.tar.gz