to build from source you need to have subversion utility ant , latest sun's jdk
and freenet-ext.jar
1) check out sourcecode from svn
svn co https://emu.freenetproject.org/svn/trunk/freenet/
2)cd freenet
3)execute
ant
it should fetch the required freenet-ext.jar from online if needed and then create a freenet-cvs-snapshot.jar file in directory lib.
4) to update your sourcecode do
svn update