I saw same sort of error when I first used the Security3Helper class in my application just like it was used in the TestDrive. I exposed the bean as a remoting destination, as follows:
On client side, if you then define a RemoteObject for securityHelper, and also ChannelSet and channel, it should work. This is what I've done:
<bean id="securityHelper" class="com.riainaday.security.Security3Helper">
Hope it helps....
<mx:RemoteObject id="securityHelper" destination="securityHelper" fault="faultHandler(event)">
<mx:AMFChannel id="amf" url="http://localhost:8080/yourApp/messagebroker/amf"/>
<mx:method name="getAuthentication" result="userHandler(event)"/>