Salue, Feles! Go to System Preferences, then to International, and then to "Input" something or "Keyboards" (should be the tab furthest to the right). There you have a list of potential keyboards you can activate. You're looking for "Latin Extended" or "USA Extended." Activate that, and then go to a program like TextEdit. In your computer's menu bar, possibly under an American flag (if you have an American computer), you'll see the "[Ae] USA Extended" keyboard. Then switch to it. Much like making an accute accent by alt+e and then a vowel, type alt+a and then any vowel, and there you go, instant macrons.
These were very helpful:
http://tlt.its.psu.edu/suggestions/inte ... latin.html
http://tlt.its.psu.edu/suggestions/inte ... ackey.html
If you want to write macrons in a post, though, it's a bit more complicated.