how to set MessageAgent's destination for remoteObject?

Hi,
i am trying to make the remoteobject work on my mxml. but i got this error

[RPC Fault faultString="[MessagingError message='The MessageAgent's destination must be set to send messages.']" faultCode="InvokeFailed" faultDetail="Couldn't establish a connection to ''"]
at mx.rpc::AbstractInvoker/http://www.adobe.com/2006/flex/mx/internal::invoke()
at mx.rpc.remoting.mxml::Operation/http://www.adobe.com/2006/flex/mx/internal::invoke()
at mx.rpc.remoting::Operation/send()
at Function/http://adobe.com/AS3/2006/builtin::apply()
at mx.rpc.remoting.mxml::Operation/send()
at Function/http://adobe.com/AS3/2006/builtin::apply()
at mx.rpc::AbstractService/http://www.adobe.com/2006/actionscript/flash/proxy::callProperty()
at RALogging1/___Button1_click()

this is the part of my mxml and the error pops up when the button is clicked. i have a javaclass "LogManager" under WEB-INF\classes\ra\logging. so i think i need to do something on some config xml. what do i need to do?

ra.logging.LogManager :
<mx:RemoteObject id="logMgr" source="ra.logging.LogManager" fault="Alert.show(event.fault.faultString, 'Error');">
<mx:method name="getList"/>
</mx:RemoteObject>

<mx:HBox>
<mx:Button label="Get Log List" click="logMgr.getList()"/>

</mx:HBox>