First you need to divide the berries into three groups of three.
Next place Group 1 one the first arm of the scales and Group 2 on the second arm of the scales and set Group 3 aside.
If the scales do not balance then you know what group of three contains, the heavier berry. Set that group aside. If the scales balance then you know the heavier berry is in Group 3.
Now take the group with heavier berry and divide into three berries. Place the first berry on the first arm and the second berry on the second arm. If they do not balance then you know which berry is the heavier. If the scales balance then the third berry is the heavier.