As outlined in the article Programming ATMEL microprocessors on a Mac, installing the AVR toolchain on the Mac can be a long and tedious procedure. Downloading, compiling and installing all the stuff can take up to 6 hours!
For a workshop I gave recently we tried to simply copy over all the binaries in /usr/local/avr/ and it worked.
Now I wrapped the whole thing into a nice installer package, see below.
echo 'export PATH=/usr/local/avr/bin:$PATH' >> ~/.bash_profile, close the window and open a new session…
For a more complete and up-to-date installer you might want to take a look at AVR MacPack