You have to wait for an invasion, which is accompanied by some sort of an announcement ("The Forces of Outworld have begun an INVASION" is the one I received upon checking this out for you) at some point after getting to the main menu. If you don't see any announcement, go to the "FACTION" sub-menu and see if "INVASION BOSS" is grayed out. If not, then an Invasion Boss is available to fight. One's available now, so get to it!
