Project's code is hosted into Subversion and here is a guide for you
How to access Flamerobin Subversion Tree http://sourceforge.net/svn/?group_id=124340
In Ubuntu for anonymous mode i did like this (for all the tree)
svn co http://flamerobin.svn.sourceforge.net/svnroot/flamerobin flamerobin/trunc
Then read the Subversion book http://svnbook.red-bean.com/
There are git and bzr mirrors (git for debian and usually is the stable branch) http://git.debian.org/?p=collab-maint/flamerobin.git to build it you need to clone it
git clone git://git.debian.org/git/collab-maint/flamerobin.git
bzr is up to date and hosted on ubuntu servers https://code.launchpad.net/~vcs-imports/flamerobin/trunk
ps: in the end we will migrate all repository to git in sourceforge and kill the svn repository see this discussion