If you're using "1602 LCD and 6 AD buttons" shield from ITeadStudio, then you need to change default pins. Below code works for me 
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// include the library code:
#include <LiquidCrystal.h>
// initialize the library with the numbers of the interface pins
LiquidCrystal lcd(8, 9, 4, 5, 6, 7);