How to run psql on Mac OS X?
Locate the psql binary. (In a terminal, run locate psql | grep /bin, and make note of the path. (In my case, it's /opt/local/lib/postgresql90/bin/, as it was installed using MacPorts.)

Then, edit the .bash_profile file in your home folder (e.g. mate -w ~/.bash_profile assuming you've textmate), and add the needed line so it's in your path, e.g.:

export PATH=/opt/local/lib/postgresql90/bin/:$PATH


After having saved the file, read the file (. ~/.bash_profile) or open a new terminal, and type psql.