You should use ~/.bash_profile. The file does not exist by default. You have to create it. You can create it with the command "touch ~/.bash_profile". "~/" is the command line shortcut for Macintosh HD -> Users -> your_user_name (commonly referred to as your "Home" folder, and the icon for which looks like a house.
In the .bash_profile, you want to add android's tools folder to the system PATH. To do that you need to add a line exporting the PATH environment variable, or append to it if it already exists.
For a brand new entry, you need to create a line that looks like this:
export PATH=$PATH:/usr/local/android/tools
For an existing line, you just add :/usr/local/android/tools to the end of the line. This is assuming of course you have the sdk installed as /usr/local/android. I did this using a symlink, which makes it easy to switch between sdk releases should I ever need to, without having to touch the .bash_profile or my eclipse configuration. I just change the symlink.
Here's 6 commands to sdk bliss (how I did it on my mac).
# download the sdk
curl -C - -O http://dl.google.com/android/android-sdk-mac_x86-1.1_r1.zip
# unzip to /usr/local
sudo unzip -d /usr/local android-sdk-mac_x86-1.1_r1.zip
# symlink for easy to change out versions
sudo ln -s /usr/local/android-sdk-mac_x86-1.1_r1 /usr/local/android
# create the .bash_profile if it doesn't exist
touch ~/.bash_profile
# edit your .bash_profile, creating or adding to the export PATH= line
/Applications/TextEdit.app/Contents/MacOS/TextEdit ~/.bash_profile
# load for the existing terminal session
source ~/.bash_profile