How do I use MATLAB to obtain a continuous state equation from a transfer function and ode45 to solve it?
You're doing pretty much everything right. The only problem is that your system is a third order system (s^3) and hence, the state x in

dx = A*x + B*u


is 3x1 but your initial condition xnot is a scalar. Replace

xnot = 100


with

xnot = [0 0 0].';


(or whatever initial condition you have) and it should work.