Are you trying to just plot an underdamped oscillator?  Or are you trying to fit beta and w0?  The below script will actually fit. 
 
Names Default to here(1);
dt = New Table("Harmonic Oscillator", 
	New Table Variable("beta", .3),
	New Table Variable("w0", .4),
	New Column("t", set values(0::50)), 
	New Column("Cos t", formula(Cos(t))), 
	New Column("Damped Cos t", Formula(
		exp(-:beta*:w0*:t)*cos(:w0*t) + random normal(0, .01) //just adding noise
	)), 
	New Column("Predictor", 
		Formula(
			Parameter({pbeta = .2, pw0 = .5}, 
				exp(-pbeta*pw0*:t)*cos(pw0*t)
			)
		)
	)
);
dt << Nonlinear( Y( :Damped Cos t ), X( :Predictor ), Newton );
 
To give you this.  notice how my actual beta/w0 was .3/.4 but I started my predictor with .2/.5

 
Is this sort of what your'e looking for?