Though TurboGears works with the Python 2.3 that comes with Mac OS X, it's strongly recommended that you upgrade to Python 2.4. Python 2.4's decorator syntax will make your life much more pleasant.
echo $$PATH
to check. If it's not there, here are some tips for changing it.If you plan to use a database with TurboGears, you will need to download and install the appropriate driver.
Mac OS X is unix-like under the hood. There are many ways to accomplish just about anything, including setting your path.
A good description for adding /usr/local/bin to your path, can be found in this MacDevCenter article. If you look at the "Laying the Groundwork" section, you'll find a discussion about setting the path just after the command "php -v" is listed.
Note that the methods listed in that article only apply to the user you're logged in as. To adjust the path for all users, you can edit the /etc/profile file.