Hello Malik,
Both the 3rd order nonlinearity and the Kerr are in fact models for Kerr nonlinearity. The difference being that the one labelled Kerr uses a model that requires the response time. The 3rd order nonlinear model simply requires the Relative linear permitivity and the 3rd order susceptibility, both of which I believe you have.
In regards to your question about the Lorentz dispersion, you would only need to enable the option if you would like the linear permittivity to be modeled using a Lorentz equation to capture material dispersion. If a constant linear permitivity is acceptable then you do not need this feature. If you wish to use Lorentz dispersion you would need to perform a fit of your epsilon_linear data as a function of wavelength to the Lorentz model to get your coefficients.
The latest version of the product provides users with the ability to load in an epsilon versus wavelength data set and have it fit to the Lorentz-Drude model.
Scott