The simplest way, of course, is using Macports:
sudo port install tesseract
But this can get a bit outdated. To get the absolute latest version from source:
sudo port install leptonica
svn checkout http://tesseract-ocr.googlecode.com/svn/trunk/ tesseract-ocr
Compile and install:
cd tesseract-ocr ./autogen.sh export LIBLEPT_HEADERSDIR="/opt/local/include/leptonica" make sudo make install sudo make install-langs
This is correct as of revision 729.