Downloading and Installing

First install Mercurial.

sudo apt-get install mercurial

Then grab the source for jpf-core and jpf-symbc packages. They are the main files for Java Pathfinder.

hg clone http://babelfish.arc.nasa.gov/hg/jpf/jpf-core
hg clone http://babelfish.arc.nasa.gov/hg/jpf/jpf-symbc

Site Configuration

Create a site configuration file with the name ~/.jpf/site.properties.

# JPF site configuration

jpf-core = ${user.home}/Software/jpf/jpf-core

# numeric extension
jpf-numeric = ${user.home}/Software/jpf/jpf-numeric

# symbolic extension
jpf-symbc = ${user.home}/Software/jpf/jpf-symbc

extensions=${jpf-core},${jpf-numeric},${jpf-symbc}

#... and all your other installed projects

Building Java Pathfinder

cd jpf-core
bin/ant test
cd ../jpf-symbc
bin/ant test
cd ../jpf-numeric
bin/ant test

Installing the Eclipse plugin

Use Eclipse 3.5+ and Java 1.6+, and this update site link: http://babelfish.arc.nasa.gov/trac/jpf/raw-attachment/wiki/install/eclipse-plugin/update/
To find out that it is properly installed, see in the preference window if there is a page for Java Pathfinder.