HELP: Error Building ArduPilot Firmware using PX4 Toolchain

I am interested in modifying the ArduPlane firmware from GitHub and flashing it onto a Pixhawk for testing/use. In the past, we have modified ArduPlane and flashed it to an APM board using Arduino Visual Micro w/ Visual Studio. Now that we’ve switched to the Pixhawk building ArduPlane has proved a bit more challenging. I followed the directions for building ArduPilot using Make files found here: dev.ardupilot.com/wiki/building- … with-make/

I decided to test the build process by using the latest version of ArduPlane that is on GitHub. I got to a place where it almost built, but the following error occurred at the end of the built process:

No rule to make target ‘…/PX4Firmware/Makefile’.

I googled this error and found a forum where a similar problem was being discussed, one developer said that the NuttX and PX4Firmware repositories have different branches or versions that needed to be checked out depending on the ArduPlane version that was being used. How, precisely, do I tell from my ArduPlane files which NuttX and PX4Firmware versions I need?