How to plot response of a non-linear state space model in Simulink?
One method is to use a Matlab Function Block to implement the nonlinear equations. You can define the inputs and outputs yourself in this block.

The body of the function block will look something like this:

function [xdot, y] = nonlinearss(x,u)

    % define  constants
    g = 9.81
    % etc...

    % nonlinear set of equations
    xdot = [x(2); g-C/m*(x(3)/x(1))^2; etc...] + [0;0;1/L]*u;

    y = x.';