Airflow around a wing creates a low pressure on the top surface of the wing and high pressure on the bottom surface. The sum of these pressures over the surface area of the wing creates the lift force to hold the plane up.
To apply pressure to a shell model, you need to apply the pressure difference between the absolute air pressure inside the wing and the absolute air pressure on the outside surface of the wing.
Suppose a plane is flying at sea level, and ambient air pressure is around 15 psi. The absolute pressure on the top surface of the wing may be around 14.4 psi, while the bottom surface of the wing may be around 15.3 psi.
Therefore, you would apply an outward pressue of 0.6 psi to the top surface of the wing and an inward pressure of 0.3 psi to the bottom surface of the wing. These pressures will have the effect of deforming the upper skin outward, the lower skin inward and bending the entire wing upward.
Whether you put a + or a – sign in front of the pressure on each surface depends on the surface normal. In Mechanical, an arrow displays the direction of the pressure. If you type a positive value and the arrow points in the wrong direction, just add a – sign and the arrow will point the opposite direction. In SpaceClaim, on the Measure tab, you can select a surface and click the Normal button to see an outward normal arrow. Positive pressure is defined as an inward normal on any surface.
Is the imported pressure absolute pressure or relative (gauge) pressure? You have to know this. If it is gauge pressure, you need to know the ambient or reference pressure.