Basic problem in ABAQUS/CAE

I am simulating a cube of sides 10 units. I have applied the
load as pressure of 16 units at top. The cube is meshed by usign a
global seed size of 5. It means, there are 9 elements and 27 nodes
(using C3D8R). I have used the material property of steel for instance.
The base of the cube is fixed and the sides are allowed to move
vertically.  Now i want to calculate the nodal force at the centre of
the cube i.e. at the centroid of the cube (not in the centre of any of
the exterior sides). 

"Sum" function in operate XY data is not appropriate, as i got a
combination of positive and negative values for nodal force in vertical
direction, if  that particular node located at the centroid is chosen.
Is there a direct way of getting it.

Thanking you,

Tanmoy Kr. Deb.