The moments like Mx/My/Mxy are reported considering a unit width of the slab. Just to give you an example, a reported value of M of say 14.83 Kip-in/in would mean a moment of 14.83 Kip-in over a width of 1 inch of slab. So when calculating the section modulus for the slab, you need to ensure that you consider a 1 inch width too. So if your slab is 6 inches thick and you consider a width of 1 in of slab, the section modulus (S) would be calculated as
S = 1/6 x 1 x t^2 in3 … where t is the thickness in inches
Substituting t = 6 in, S = 1/6 x 1 x 6^2 = 6 in3
M=14.83 Kip-in … the /in portion is only to tell you that this moment is reported over a width of 1 in of slab
So bending stress due to M = M/S = 14.83/6 = 2.47 Kip/in2
The Sx Sy etc. are also reported in Kip/in2 and hence to get combined stress, you can do an algebraic summation.