Under "User Defined Functions" there is a fcn block which allows you to enter equations to generate an output variable as a function of input variables. The input can be either one signal called u, or a signal vector with each input called u(1), u(2) ... u(n), where n is the number of signals in the vector.
So, as an example, if you wanted to multiply two inputs, you would write u(1)*u(2) in the equation for the fcn block.