Combobox not save to propertyRef variable

I create a combobox in the graphic builder like in this topic
http://www402.abbext.com/How-make-Drop-box-Combo-box-800xA-PG2-graphics-q12184.
aspx#commentsList12184

Then I tried to show a selected value in the text box by use propertyRef#Value but nothing show up. I also tried convert to string but not working.

Am I do this the right way?

FYI: I used propertyRef#IsWritable and it return False. Don't know if there's anything to involve with this.