Apple installs the Garamond TrueType collection (.ttc) by default when the operating system is installed. This location is not, however, in the normal search path for fonts.
Click on your Desktop to make Finder active, and then press shift+command+G to open the Go To Folder… dialog. Copy and paste the following into the Go To Folder dialog, and then click GO.
A Finder window will open in the above folder. Locate Garamond.ttc, right-ciick on it, and select the secondary menu item "Copy Garamond.ttc." Press shift+command+G again, and copy/paste the following, followed by clicking GO again.
This is your local font library. Right-click, and from the secondary menu, select Paste "Garamond.ttc."
Garamond should now appear on your Outlook font menu. If it doesn't then reboot, and look again.