Use bwselect() in the Image Processing Toolbox.:
BW2 = bwselect(BW,n) displays the image BW on the screen and lets you select the (r,c) coordinates using the mouse. If you omit BW, bwselect operates on the image in the current axes. Use normal button clicks to add points. Press Backspace or Delete to remove the previously selected point. A shift-click, right-click, or double-click selects the final point; press Return to finish the selection without adding a point.
