In recent versions of OS X (at least since 10.5), you can type the accented letters of Esperanto very easily, without downloading anything or using a website, as long as the language of your system is English.
To prepare your computer (required only once):
1. Open “System Preferences…” from the Apple menu.
2. Click on “Language & Text”, or “International” (depending on your version of OS X).
3. Click the tab “Input Sources”, or “Input menu”. You will see a long list of languages (keyboard layouts, in fact). If you live in Australia, chances are “Australian” is already selected.
4. Scroll down to “U.S. Extended” and select it.
5. Scroll up again and deselect “Australian” (or whichever keyboard layout was selected before)*.
6. Close the window.
Then, to type the accented letters, do the following:
For ĉ, ĝ, ĥ, ĵ or ŝ, press option-6, and then c, g, h, j or s. (This also works for the capital letters.)
For ŭ: press option-B, and then u. (This also works for Ŭ.)
(The reason for “option-6” is not surprising, because above “6” on the keyboard is “^”. “B” is obviously from the word “breve”, which is what the ˘ symbol is called.)
* You can keep your original keyboard layout selected if you like; but if you do, you will need to switch between that and “U.S. Extended” as needed, by typing ⌘-space.