Macronised Vowels for Windows Users
For Windows users, Microsoft's New Zealand Māori keyboard definition is included in Windows XP Service Pack 2.
To set up the keyboard definitions, follow these steps:
1. Click "Start"->"Settings"->"Control Panel"

2. Choose "Regional and Language Options"

3. Click the "Languages" tab

4. Under "Text services and input languages" click the "Details..." button
5. Under "Installed services" click the "Add..." button

6. In the "Input language" drop down box, select Maori then click OK

7. Under "Default input language" select "Maori - Maori" as the default language

8. Click OK until you are out of "Regional and Language Options"
Alternative

Alternatively, if you don't want Maori to be the default input language, or you would like to change your input language easily, you can use the language toolbar in the bottom right of your screen. If this does not show up, you can right-click on empty space on the task bar, select "Toolbars", and check the "Language bar" option.
Now when you want to enter a macronised vowel, press the ` key (top left corner of your keyboard) followed by the vowel. If you want to enter the ` character, press the key twice.