"mvn install" at the same directory as the topmost pom.xml
After you install via maven:
1) Update build.properties with at least the directory for your local maven installation
2) You can go to the BroadleafCommerceDemo directory and execute "ant jetty-demo" to launch the demo with admin.
If you want to do this in Eclipse, then you'll need to make sure you have the m2eclipse plugin and mount your subversion checkout as a maven project. This will allow you to mvn install from Eclipse. You can also mount the build.xml file in the BroadleafCommerceDemo directory in your Eclipse ant view and execute the jetty-demo task from there.
You don't need to configure a db for the demo - it will run on HSQL. Of course, this is for demo purposes only.