The SITL (Software In The Loop) simulator allows you to run APM without any hardware. It is a build of the autopilot code using an ordinary C++ compiler and giving you a native executable that allows you to test the behaviour of the code without hardware.
Refer to the APM wiki for installation instructions.
Install just the command line tools for OSX including GCC then get modules installed to Python:
sudo easy_install pip sudo pip install pexpect sudo pip install pyserial
Install MAVLink and MAVProxy:
sudo pip install pymavlink MAVProxy
(or) install MAVLink and MAVProxy from source
git clone https://github.com/tridge/MAVProxy git clone https://github.com/tridge/mavlink
run this setup command in both directories
sudo python setup.py build install
Get the APM branch that compiles in Mac OS (this commit is relevant):
git clone https://github.com/erlerobot/ardupilot cd ardupilot git checkout macos cd ArduCopter make configure make sitl