TeXShopWhere's My Stuff?
Unix has a different philosophy about TeX than earlier distributions on Mac OS 9. Under Unix, a user's personal files, additional style files, and modified standard TeX files are kept separate from the multitude of files in the standard distribution. This makes it easy to upgrade TeX without modifying the user's changes, and it makes it easy for users to remember changes they have made to the system.
As explained on other web pages, TeXShop supports multiple TeX distributions. The exact location of TeX on your machine will depend on which distribution you have installed. The standard TeX Live distribution described under the "Obtaining" tab is in /usr/local/texlive/2009. Gerben Wierda's gwTeX is in /usr/local/gwTeX, and the old teTeX distribution is in /usr/local/teTeX. These locations can be searched with the Terminal application but are not visible in the Finder. A symbolic link to the currently active distribution is in /Library/TeX/Root. Users can use the Finder to examine files in their active distribution by going to this location. 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 file in ~/Library/texmf/bibtex/bib or in a subfolder of this folder. Etc.