the most straightforward way to calculate the three diagonal components of the dielectric function with EXCITING is to perform excited state calculations. Our code implements both Time-Dependent Density Functional Theory and Many-Body Perturbation Theory. I would strongly suggest you to give a look at the tutorials under the sections "Time-Dependent DFT" and "Bethe-Salpether Equation" to become more familiar with the input files. As you will see, you don't need to specify by hand the optical components through the element "optcomp". Once you perform this kind of calculations you immediately obtain the diagonal optical components of the dielectric function: they are stored in files named EPSILON_*_OCii, where ii=(xx,yy,zz). With EXCITING we also provide some scripts to easily process these files.
If you do not have more elaborate requirements I would recommend you to use the elements specified in the input files of the tutorials: they not only represent the standard procedure to compute the dielectric function but they also contain more advanced features, e.g. the possibility to tune local field effects, which will be further maintained and developed also in the next version of the code, which will be released soon.