In order to apply distributed stress to the boundary go to "Initial Condition" tab-> then click on "Nodal Apply"->select stress type (i.e. sxx, syy or nstress)->specify magnitude->use mouse to select the boundary nodes you want to apply stress at
here is an example of distributed vertical stress to boundary:
applybc("syy",-100.0,"xlim",-46.301,27.313,"ylim",-1.117,1.218)
The attached screenshot illustrates the procedure for applying the distributed stress.