I'm very confused as to where the center of symmetry is for the PbO (litharge form). I know that it is in the P4/nmm space group, and that it's centrosymmetric, but beyond that I'm clueless. Thank you.
The best thing to do is load up the CIF at the Crystallography Open Database (COD), e.g. http://www.crystallography.net/1010977.html?cif=1010977
JSmol used for the visualization lets you visualize the symmetry and adjust the packing. In this case, look for the inversion center, e.g.:
The inversion center itself is the yellow ball. But it's probably easier to see it in the inverted Pb centers I've circled in yellow. You can see that there's an exact inversion center there.