I had problems like this after installing DBD-Pg on Linux & on Cygwin 
when the library files for Postgres itself weren't getting found in 
LD_LIBRARY_PATH. Make sure that the system can find its way to 
/usr/local/postgres/lib (or whatever - it should have files such as 
libpq.so in it). Tweak /etc/ld.so.conf (or the equivalent on OS X - 
sorry, I'm not familiar with how library paths work on that) or 
LD_LIBRARY_PATH, as you need.

HTH
alex

culley harrelson wrote:
> I am having trouble with DBI-Pg on os X (ruby 1.6.7).  I installed it 
> successfully using config --with=dbi,dbd_pg but when I try to establish 
> a connection I get:
> 
> 704 $ ./test.rb
> /usr/lib/ruby/site_ruby/1.6/dbi/dbi.rb:502:in `load_driver': Could not 
> load driver (No such file to load -- postgres) (DBI::InterfaceError)
>         from /usr/lib/ruby/site_ruby/1.6/dbi/dbi.rb:392:in 
> `_get_full_driver'
>         from /usr/lib/ruby/site_ruby/1.6/dbi/dbi.rb:372:in `connect'
>         from ./test.rb:4