pressure boundary condition in elasticity problems

Hi, this is probably a silly question but I don't seem to be able to find an answer. I want to calculate stresses within a loaded block, like similar to http://sfepy.org/doc-devel/primer.html but instead of force on a node I want to be able to specify pressure on the whole boundary in Pa.

It seems to be possible to do by specifying "load" in the material properties but it seems to be the force in N instead of pressure. Is there some other way or should i calculate the force per node and then use the calculated value in "load" ?

Thanks in advance.