TeXShopInstalling TeX Live
If you downloaded MacTeX-2008 as recommended under the "Obtaining" tab, double click MacTeX to begin the installation process.
Follow the same procedure if instead you obtained BasicTeX.
Some History
Users interacting with TeX using a front end program are sometimes unaware of the vast support machinery acting invisibly behind the scene. This machinery consists of Donald Knuth's command line program, which does the actual typesetting, and of an enormous number of fonts, macro packages like LaTeX and ConTeXt, style files, documentation, configuration files, and the like. The enormous collection of programs and support files is called a TeX Distribution.
For a number of years, the standard TeX distribution on Mac OS X and GNU/Linux was teTeX, maintained by Thomas Esser. On the Macintosh, this distribution was enhanced by Gerben Wierda, who wrote a program called i-Installer to download his enhanced version from the network, to configure it, and to upgrade it periodically.
Several years ago the TeX Users Group introduced an even more extensive distribution called TeX Live, for Mac OS X, Windows, GNU/Linux, and various BSD Unix systems; the principal authors are Sebastian Rahtz, Karl Berry, and Staszek Wawrykiewicz. In May, 2006, Thomas Esser announced that he would no longer support teTeX, and suggested that users move to TeX Live. This caused a scramble throughout the TeX world.
As a consequence of this history, several different TeX Distributions are commonly used on Mac OS X. The most popular distribution is TeX Live, but Esser's teTeX is still used by some, a newer gwTeX by Gerben Wierda is used by others, BasicTeX and and subsets of TeX Live in Fink and MacPorts are also used.
Multiple Distribution Support
Gerben Wierda and Jerome Laurens recently designed a data structure to support multiple TeX distributions on a machine. This data structure is installed by the MacTeX packages. The data structure is placed in /Library/TeX and consists of some carefully designed symbolic links to installed distributions. Using this structure, teTeX, gwTeX, TeX Live, and Basic TeX easily coexist on a machine.
After installation, a new Preference Pane can be found in Apple's System Preferences. The pane, called "TeX Distributions", lists installed TeX distributions; the active distribution is hilighted. Using the pane, a different distribution can be made active. When that happens, all GUI applications are reconfigured automatically, and PATH and MAN variables are reset for command line interaction with the new active distribution.
The data structure is ingenious; it does not modify TeX distributions in any way. Switching distributions actually modifies only a single symbolc link. A new link, /usr/texbin, is created which points to the binary directory of the active TeX distribution. GUI applications need to be reconfigured for the data structure, but after they are configured once, they will work without modification in the future. TeXShop, BibDesk, and LaTeXiT all use /usr/texbin automatically.
The data structure installed by MacTeX understands the older teTeX distribution. So if you were using teTeX in the past and you install TeX Live, your teTeX will remain completely unchanged, and will be listed along with TeX Live in the preference pane. TeX Live 2008 does not modify TeX Live 2007, so if you are in the middle of an important project, you can safely install TeX Live 2008 knowing that you can switch back to TeX Live 2007 with a single button click if you encounter problems with the 2008 version.