Hi Lynn,
the easiest is to use the function flow line (which is called by plot_streamlines). You can try something like that:
flowpath=flowlines(md.mesh.elements,md.mesh.x,md.mesh.y,md.initialization.vx,md.initialization.vy,x0,y0);
expwrite(flowpath,'Flowlines.exp');
exp2shp('Flowlines.exp','Flowlines.shp');
I hope this helps!
Mathieu