To obtain and build Emacs 21 on Mac OS X, basically all you need do is to follow the instructions at the Emacs CVS repository to obtain the source code and then follow the instructions in the files INSTALL-CVS and mac/INSTALL to build Emacs. Since the code checked out by CVS does not contain the compiled Lisp files, you must run make bootstrap instead of just make. Note that the Emacs developers change this code in the CVS repository all the time! As such the most current version of code you checkout may not work correctly or may not even build at all. You should be prepared to put in some effort yourself to build and run it. To find out what the latest problems might be in the source code, read the emacs-devel mailing list. If you are a beginner, please don't post questions there; post to gnu.emacs.help instead. Also please read the FAQ and the News sections before you post for help. Emacs for Mac OS X has been reported to build and run on both Mac OS X 10.1, 10.2, and 10.3. You will of course need the Developer Tools for your version of OS X, which you can download free of charge from the Apple Developer Connection website. You will also need to download and install the latest GNU texinfo package as described in the mac/INSTALL file. The sequence of commands to obtain and build texinfo and Emacs (assuming that you already have Developer Tools installed) should look something like the following. You may need to modify them for your system. If you are building on Panther, skip the steps for installing texinfo since Panther already installs it. 
  curl -O ftp://ftp.gnu.org/gnu/texinfo/texinfo-4.6.tar.gz
  tar zxf texinfo-4.6.tar.gz
  cd texinfo-4.6
  ./configure
  make
  sudo make install

  cd ..
  cvs -d:pserver:anoncvs@subversions.gnu.org:/cvsroot/emacs login
    (type RETURN when asked for a password)
  cvs -z3 -d:pserver:anoncvs@subversions.gnu.org:/cvsroot/emacs co emacs
  cd emacs
  ./configure --with-carbon --without-x
  make bootstrap
  sudo make install
To run Emacs, double-click the Emacs.app icon in the Finder or type the full pathname to .../mac/Emacs.app/Contents/MacOS/Emacs from a terminal window. To build Emacs to run under X Window, change the ./configure line to 
  ./configure --with-x --without-carbon
Note that to run Emacs under X Window, the environment variable DISPLAY needs to be set appropriately. For more complete instructions and specialized methods of building Emacs, such as building it as an application bundle, please read the file mac/INSTALL. The instructions above are provided merely as a courtesy. They do not replace studying the install files and manuals.