A multiplier takes its two input signals and multiplies them. If the inputs are of type matrix it will calculate the matrix products. When declaring this block you have to indicate the two types of the two input signals as well as the type of the output signal, e.g.:

Mul<Matrix<2,1,double>,Matrix<1,3,double>,Matrix<2,3,double>> m1;
Mul<> m2;   // all types will be double