TeX Live, and lots of other software, defaults to installing under /usr/local on all Unix systems, including Macintosh. For example, the TEXMFLOCAL tree for local additions is /usr/local/texlive/texmf-local by default. There are some small Mac-specific complications.

• Problem 1 -- The Finder doesn't show /usr/local by default: There is a "Go" menu in the Finder; select "Go To Folder" in this menu and type /usr/local in the short dialog which appears after the selection. After that, the location is shown in the standard way and the Finder works as usual.
• Problem 2 -- Moving stuff into /usr/local/texlive/texmf-local and its subfolders: Drag and drop works as usual, but a panel will come up asking for a password. The operation completes after your user password is typed.
• Problem 3 -- Removing stuff from these locations: Same story.
• Problem 4 -- Editing files that are in these locations: One easy solution is to drag the file elsewhere, edit it, and drag it back. Other methods work as well.

To manipulate /usr/local items, sudo is what's generally needed, and any Mac user with admin privileges can use sudo with their own password. If only one person uses a machine, that person automatically has admin privileges.

Hope this helps. Please write mactex-support@tug.org with Mac-specific questions or problems.