The ANTLR v4 website provides a nice and simple quick start for OS X developers. If, however, you find yourself requiring the richer feature set of ANTLR v3, the installation process is not quite so transparent.
Borrowing heavily from the v4 quick start:
cd /usr/local/lib wget http://www.antlr3.org/download/antlr-3.5-complete.jar export CLASSPATH=".:/usr/local/lib/antlr-3.5-complete.jar:$CLASSPATH" alias antlr3='java -jar /usr/local/lib/antlr-3.5-complete.jar'