since there are some changes recently in software it might help to get an update about
how to load the current development version on to your APM.
If you just want to have a quick start i strongly advise you to upload the latest stable version (2.20)
from the Mission Planner (Firmware Tab).
If you want to tinker with code you should load the latest version from here:
For uploading the development version you need the modified Arduino IDE downloaded from here:
Just download both archives and unpack both in seperate directorys.
Start the downladed Arduino IDE and open the preference window.
Select the directory where you have unpacked the software archive master.zip (probably called "ardupilot-master")
In the Directory should be the "APMrover2", the "libraries" folder and some others.
Close Arduino IDE to save changes and start it again.
You should now be able to load the APMrover2 sketch.
Select your Hardware from ArduPilot menu and your board from Tools/Board menu.
Now you should be able to compile without errors.
If you're having trouble or Arduino shows an error you should probably read more about usual occuring problems here:
I have made a quick video for demonstation purpose also.