there are one very good book titles as "The FDTD method for electromagnetics with MATLAB Simulation" by Atef Elsherbeni and vensyl Demir .
this book published by Scitech publication .
in this author provide matlab code for every step by step to solve problem in FDTD.
Sullivan also has a book, the exact name of which escapes me. Google Sullivan FDTD and you should find it. It provides code in C, but that's very easily adapted to MATLAB. Sullivan is a very basic tutorial so should be helpful.