Commenting with some updated information from 2018.
Executable Binaries for Platform tools are made available for mac by Android here: https://developer.android.com/studio/releases/platform-tools.html
Download these to your mac. Place them in a directory e.g. I placed in ~/Software/platform-tools
If you have root access, the easiest way I have found on a mac is to add your directories to the list in /etc/paths. I like this way because after a few additions the $PATH starts to look too messy and hard to read, the /etc/pathshas everything in separate line, clean and organized. The downside is that you require root access.
$ cat /etc/paths # check contents of the file
$ sudo nano /etc/paths
Paste the full path of your platform-tools directory (something like /Users/GodZilla/Software/platform-tools/adb) at the end of this list and save. Quit and Open terminal again and check if it sees your platform-tools directory.
$ which adb
/Users/GodZilla/Software/platform-tools/adb
$ which fastboot
/Users/GodZilla/Software/platform-tools/fastboot
If you don't have root access, just add the platform-tools directory to $PATH in your .bash_profile (or .zshenv if you use zsh) as other users have suggested.