MATLAB does not support the i, j, k quaternion "complex" numbers. The i and j in MATLAB are strictly the ordinary imaginary numbers.
To use quaternions, unless you have special class code for this, typically you would just represent them as ordinary 4-tuples. E.g.,
q_1 = [MM;A;B;C]
q_2 = [NN,D,E,F];
Then if you want to add them it is simple addition: