This is a little more complicated, as its Apple after all! Please note, it does not work for secure areas such as password inputs. I have not found a work around yet.

Download the repo. It will install in /usr/local/bin/keylogger.

Install it:

$ git clone https://github.com/GiacomoLaw/Keylogger && cd keylogger
$ make && make install

It will log to /var/log/keystroke.log. This may require root access, but you can change that if you want. Set where you want it to log:

$ keylogger ~/logfile.txt
Logging to: /var/log/keystroke.log

Want to make it start on system startup?

$ sudo make startup

That will run it on startup.

$ sudo make uninstall

Will uninstall the program, but not the logs.