I believe the installer assumes you are running under the bash shell. If you are running under csh or tcsh you will need to modify your .login file.
If the installer doesn't set the path environment variable you will need to edit your .login file so the path gets set at login.
Open Terminal
$ cd ~/
$ vim .login
once in vim use the arrow keys to navigate to the bottom of the document.
Once at the bottom press the 'a' key to enter edit mode.
add the following line to the end of the document
set path = ( "/usr/local/msp430-toolchain/bin" $path )
Now hit the esc key, then hit the colon (command mode), and finally type in 'wq' (to write the file out and quit) and press enter.
If you would rather use a Macintosh editor with a standard Aqua (GUI) interface, see TextWrangler on my Text Editors Page.
Log out and log back in and your changes will be in effect.
