I have build 252 installed.  I seem to be having trouble getting a simple mask to work.  I think I must be missing something obvious.

I place a clip on the time line. 

I also place a solid color on a track below it.

On the top video clip I select the Pan/Crop icon and go to the bottom of that window to click on the track to bring up the Mask settings.

I select the rectangle tool and draw a rectangle in the window but nothing gets masked out nor does the solid color track show through.  Changing the mode from positive to negative or changing  "apply to FX" to yes makes no difference.

There used to be a mask enable/disable check box but I fail to see that.