Where's My Stuff?
The GUI applications used to interact with TeX are installed in /Applications/TeX. This location contains a file named "READ ME FIRST" which explains how to get started using TeX and LaTeX with TeXShop. An additional document, "What Is Installed'", explains in detail exactly what was installed by MacTeX. For most users, the GUI apps will suffice and it will not be necessary to directly examine TeX Live.
MacTeX installs TeX Live and Ghostscript in /usr/local, a standard Unix location for additions to the system. In particular, TeX Live is placed in /usr/local/texlive/2011, where this last item is the release date of a particular version of TeX Live. BasicTeX is in /usr/local/texlive/2011basic.
These locations are not visible in the Finder by default. A symbolic link to the currently active distribution is in /Library/TeX/Root. Users can use this location to examine files in their active distribution. There are a lot of files!
Personal files should be stored in ~/Library/texmf. (Recall that ~/Library is the library folder in your home directory, while /Library is a system folder analogous to /Applications.) You will have to create the subfolder "texmf." When TeX needs to open a file, it searches ~/Library/texmf first, so if you modify a standard TeX file and place it there, the modified file will be used.
The folder structure inside ~/Library/texmf should mimic that of the texmf trees in your TeX distribution. This is easier than it appears. TeX will locate any file in ~/Library/texmf/tex or in a subfolder of this folder; LaTeX will locate any file in ~/Library/texmf/tex/latex or a subfolder of this folder. Bibtex will locate any .bib file in ~/Library/texmf/bibtex/bib or in a subfolder of this folder, and any .bst file in ~/Library/texmf/bibtex/bst or in a subfolder of this folder. Etc.