to add masses, set the mass, dx/dy (delta x and delta y, that is - velocity.).. and click somewhere. Shift click to make a fixed (unaffected by gravity) mass. Note that a fixed mass can have a dx/dy which means you'll have an object unaffected by gravity but moving!
Velocity vectors are drawn in red.
sorry about the blinking. i'll fix it later.
srl@monkey.sbay.org
oh yeah, source is here