I set the PATH,but it didn't work.I find other way to solve it. (OSX 10.10 & laravel 5.2)
1) find the executable file:
~/.composer/vendor/laravel/installer/laravel
2) give execute permissions:
chmod +x ~/.composer/vendor/laravel/installer/laravel
3) make a soft link to /usr/bin:
sudo ln -s /Users/zhao/.composer/vendor/laravel/installer/laravel /usr/bin/laravel