2) Next, you need to download and install a Syriac Unicode keyboard. There are several Syriac keyboard layouts you can find online, like this one.
3) After you have downloaded the keyboard, you have to move it to the correct folder:
Macintosh HD -> Library -> Keyboard Layouts
4) Once the keyboard layout is in the right location, you need to activate Syriac as a language option in System Preferences:
System Preferences -> Language & Text (or perhaps International, depending on the OS version)
In Language & Text, click the "Input Sources" tab, then scroll down and select Syriac.
And if you have never used a Unicode language, you should select the "Show input menu in menu bar" box at the bottom of the screen. This will show a flag icon in your top menu (at the right) which will allow you to selct different languages you have selected easily. You can also set keyboard shortcuts to swtich between languages.
Note: If Syriac does not appear as an option in your list of languages in the Input Sources screen, then your computer has not recognized the keyboard layout. Make sure it is in the correct folder, and then try restarting your computer. If it still does not work, try finding another Syriac keyboard layout online.
Part 2: Using Syriac on your Mac
1) Internet browsers
Mozilla Firefox is the only web browser that fully supports Eastern unicode scripts, so you can read Syriac on websites and even type in Syriac on your websites. Currently, while Chrome and Safari support unicode, they do not support right to left, connecting scripts.
Mellel is the best word processor for using Syriac (and other Eastern scripts like Hebrew and Arabic) on a Mac. And as an added bonus, it is quite affordable (especially if you are a student because they offer a generous student discount).
Microsoft Office for Mac
Regarding Microsoft Office, and specifically Microsoft Word, there is bad news and good news. First the bad news: Microsoft Office for Mac does not fully support right-to-left typing or connecting scripts. (Note: this has nothing to do with Mac OS; Microsoft just does not make the Mac version of Office do everything that the Windows version can do. The lack of right-to-left support is only one of a number of features not available on the Mac Version of Office.)
Now the good news: the newest version of Office for Mac (2011) has demonstrated limited success with using Eastern scripts (including Arabic, Hebrew, and Syriac), despite the fact that they are not fully supported. Specifically, this means that although you cannot create a setting to make the Eastern scripts work correctly, you can migrate the correct setting from another word document that contains Syraic text. Here is a word document you can download that contains Syriac text (typed on a Mac!). You should be able to save the "style" on your own computer and then use this style in the future. (Note: this is a paragraph style, so if you are trying to type a single Syriac word/phrase within a paragraph of other text, you will have to play around with the styles.)
If you don't want to use either Mellel or Word, you can use open office software (like NeoOffice) or Nisus.