Problem solved: As rayreng and carandraug suggested, I needed to create a struct.I did this via a executable I run from terminal (or, alternatively as .m from Octave):
#! /usr/bin/octave -qf
load DHM
surfaceTopography.xllcorner = 944452;
surfaceTopography.yllcorner = 6452389;
surfaceTopography.cellsize = 1;
surfaceTopography.nrows = 144;
surfaceTopography.ncols = 144;
surfaceTopography.data = DHM;
calculateTerrainParameter(surfaceTopography,30,20,7)
Thanks again!