He is either doing it outside of a function or on initialize and the Object isn't filled yet because the user isn't signed in.
code:
var myOldPsm;
function OnEvent_Initalize(bMessengerStart) {
if (Messenger.MyStatus < 2) return false;
myOldPsm = Messenger.MyPersonalMessage;
}
function OnEvent_SigninReady() {
OnEvent_Intialize(true);
}