Not sure if that is what you are looking for, but here is how you could do it in Control System Toolbox:
>>a=1; % change to desired value
>>b=4; % change to desired value
>>s=tf('s');
>>plant=-a/(s^2-b);
>>C=pidtune(plant,'PID'); design pid controller
>>step(feedback(C*plant,1)); plot the step response of the closed-loop system
>>pidtool(plant); % open the GUI for interactive tuning
HTH. Arkadiy
