Hi,
The equation ruling Kerr nonlinear materials is described in the technical background manual page 24. The values are intrinsic to the material you are trying to simulate. You can either find them experimentally or look them up in scientific articles or databases.
To change the input power:
-go to the input field properties window (by double-clicking the input source in the project browser or double clicking the red line/point in the designer window)
-Open the 2D or 3D transverse tab (depending on your type of simulation)
-Change the amplitude or power values at the bottom of the panel
Best regards