(Note that for convenience, "z" has been used instead of "\zeta" and "w",
instead of "\omega_n".)
First write the equation in the state space form:
x'1=x2
x'2=2zwx2+w2+u(t)
y=x1
This is in the state-space form. Now rewrite it in the ``standard" form
treating w2 also as part of the input (in other words, redefine
the "new" input as v(t)=w2+u(t)). So we have
x'=Ax+Bv
y=Cx+Dv
with A=[0 1;0 2zw], B=[0;1], C=[1 0], and D=[0].
Then use the standard state-space block in SIMULINK. Note that the input
to the state space block is now u(t) + w2 - so you will need to
use a summing junction to add these two up and then send send the sum as input to the state space block.