Two issues here:
- If you look at the example syntax right above where you typed, you need to use MATLAB-style indexing with parentheses. So you want u(1) instead of u[1], for instance.
- Simulink muxes do not implement "nesting"; instead, they stack all signals vertically. In your example, the output of the second Mux should simply be a 4-element array.
By the way, if all you want to do with that Fcn block is extract individual elements of a multidimensional signal, you can also use the Selector block.
Sebastian
