Take the Pen tool, make a point “A”, then, go further in the comp window, and make a “B” Point, and just after you made your point “B”, !dont let go your left mouse button!, move your mouse to any direction and it will make a bezier curve with the line.
Or, draw 2 points with the mask, like above, and press G four times in a row so you have the Bezier Tool. Then, just take that tool and press your “A” and/or “B” points, then move it to any directions (without letting go your mouse button, of course).
Or check RotoBezier in the toolbox.
Hope that helped.
Justin Productions
Tangerin01@hotmail.com
Adobe After Effects 6.5 Professional