Save the attachment to your Mac. It will probably end up in Downloads. Open iTunes, then go to File/Add to Library. A Finder window will open when you click on Add to Library. You can navigate to the file, and click Open. It will then be added to your iTunes library. I just tried it with some music I had on a memory stick.
If you have trouble finding the file, turn on Show File Extensions (which in my opinion should always be on regardless of which computer you use) and do a search in Finder for .m4a. You should find the path to it. I'm using Mountain Lion. Earlier versions of OS X may be different, although I wouldn't think they would be appreciably different. This is almost exactly the same way you would do it in Windows.