To checked out a branch from svn do something like:

svn co http://www.openscenegraph.org/svn/osg/OpenSceneGraph/branches/OpenSceneGraph-osgWidget-dev OpenSceneGraph-Experimental

Then to sync this branch with the main svn/trunk do:

cd OpenSceneGraph-Experimental

svn merge http://www.openscenegraph.org/svn/osg/OpenSceneGraph/branches/OpenSceneGraph-osgWidget-dev http://www.openscenegraph.org/svn/osg/OpenSceneGraph/trunk .