Just run ant from the main directory where you see the build.xml file. Use the build target:
ant build
Or, for old people like Terence, you can do the following. Make sure the following two jars are in your CLASSPATH
antlr-3.0/lib/stringtemplate-3.0.jar antlr-3.0/lib/antlr-2.7.7.jar
then jump into antlr-3.0/src directory and then type:
$ javac -d . org/antlr/Tool.java org/antlr/*/*.java org/antlr/*/*/*.java
Takes 9 seconds on my 1Ghz laptop or 4 seconds with jikes. Later I'll have a real build mechanism, though I must admit the one-liner appeals to me. I use Intellij so I never type anything actually to build.