Good news, I have found the solution to your problem after examining the code you provided
Make this change in the file '_FillMoviesWindowData.pbi' :
;replace the first line for the second one
;SetGadgetText(#Gadget_ViewDate, \ViewDate) ;this sets the mask x)
SetGadgetState(#Gadget_ViewDate, ParseDate("%dd/%mm/%yyyy", \ViewDate))
You were setting the mask of the gadget to be the same as the date, which wouldn't allow any editing because when the date "01/03/2012" is used as a mask it doesn't allow any values to be entered (no %d, %m, or %y).