Thanks for your interest in the openHAB project!
Building and running the project is fairly easy if you follow the steps detailed below.
The openHAB build infrastructure is based on Maven in order to make it as easy as possible to get up to speed. If you know Maven already then there won't be any surprises for you. If you have not worked with Maven yet, just follow the instructions and everything will miraculously work ;-)
What you need before you start:
Make sure that both "mvn" and "hq" commands are available on your path
Check out the project sources from Google code:
hg clone https://openhab.googlecode.com/hg/ openhab
You will now have all required sources in the openhab subdirectory.
To build openHAB from the sources, Maven takes care of everything:
The build result will be available in the folder openhab/distribution/target. Both the runtime as well as the designer zips are placed in there.