The Māori keyboard was accidentally turned off in the 0.1 version. This page explains how to turn it on..
Load the GNOME Keyboard Applet by right-clicking on the panel (the bar at the top or bottom of the screen) and selecting 'Add To Panel', 'Keyboard Layout SWitcher'.
The keyboard switcher will appear. Click on it to switch to the Māori keyboard; you will see the NZ flag next to the letters 'mi'.
Now, anywhere you can type, you should be able to hold down the right <ALT> key while typing a vowel to get macrons. The letter ā is typed with the <right-hand ALT>+a combination.