Go to 'Post Processing' then choose 'template-based processing', choose S-parameters in drop-down menu in first row, later choose "Extract permittivity and permeability from S-parameters" in second drop-down menu. Clicking 'Evaluate' will give you both of these parameters including refractive index as well.
First, you need to solve the MTM structure in TEM mode (TE or TM mode will not give refractive index directly as far I know). Then you should go to the "Post-Processing" Option on the top bar. Then select "Result-templates". In the Result templates option, select "S-parameter", and Select "Add new post processing steps". Then Select "Extract Permittivity and Permeability" and then Click to "Evaluate". The refractive index will be found on "Tables"> "1D Results". The Screenshot of my current project is given below. I hope this satisfies your question. Thank you.