matrix1.setcell1d(x, n_array); // where n_array is int float or double
(but probably you've figured it by now)
I've been using setcell a bit at first, but realized that it's actully quite slow. If you wanna manipulate large amounts of data it's advisable to use the copy comands supplied by the JitterMatrix object. i.e.
copyArrayToMatrix, copyArrayToVector and copyArrayToVectorPlanar etc..