This helpful website shows you how to get that little flag that enables you to use the U.S. Extended keyboard:
Now that you have that set up, you're ready to add those pesky macrons. In your document press the Option key, then the A key. Your macron will appear (a little higher and longer than the finished product will be.
Then just type whatever vowel you want to use.
Now, for those of you who haven't bought a Mac yet, the aforementioned website also gives instructions for Windows users and even HTML. : )
The overdue, final post of my Iditarod series.