Action support rerender not showing Outputpanel

Switching between Radio Option should show and hide Outputpanel. I got this working with following code:

     <apex:selectRadio id="benefitradioid" required="True" value="{!selectedTargetBenefit}" style="float:left;">
          <apex:selectOption itemLabel="SECOND" itemValue="secondvalue"/>
         <apex:selectOption itemLabel="THIRD" itemValue="thirdvalue"/>

         <apex:actionSupport action="{!setSpecifiedBenefitLevel}" reRender="myPanelID"/> 
</apex:selectRadio>

<apex:outputPanel id="myPanelID" rendered="{!showBenefitLevel}">
     <apex:inputText ----********************>
</apex:outputPanel>

and in Controller :

public void setSpecifiedBenefitLevel(){
   System.Debug('----'+selectedTargetBenefit);
    if(selectedTargetBenefit=='secondvalue'){
        showBenefitLevel=True'
    }
}

My debug logs doesn't show anything. What could be the issue or any changes I need to do ?