Hi,
Go to the aMule.org home page (
http://www.amule.org). Click on the "Download" button on the left side. In the resulting list of files, scroll down until you find "aMule-Mac-2.0.3.zip". Click on that. Then, click on the little picture of a floppy disk.
The aMule zip file should begin downloading. Depending on your browser, it may be unzipped for you automatically when the download completes. If not, just double-click on the downloaded zip file.
Now you've got aMule.

Just double-click it to run it.
If you have further questions, check the
Mac OS X forum to see if they've already been answered. If not, feel free to ask.