You need to have Apache Ant installed, and <ANT_HOME>/bin in the classpath.

To retrieve the third party dependencies, run the following from the command line:

ant retrieve

This will populate the /lib directory with all the dependent jars.
Alternatively your IDE might support ivy, so install the plugin, set the
ivysettings.xml and ivy.xml, and it should create a classpath container with the jars resoved by ivy.