Typically in a tragedy, the "Anticipation Stage" happens when the tragic hero is somehow unsatisfied; this stage also usually involves the moment when the hero discovers a goal that he (or she) can begin pursuing. The thing is, Agamemnon is an unusual tragedy because the tragic hero is offstage for most of the play. Thus, even though in a normal tragedy, the different stages of the plot would all be directly relevant to the hero, in Aeschylus's play they are distributed among the other characters of the play. Thus, the Anticipation Stage in Agamemnon happens when the hero is not present; you could say that the Watchman and the other characters of Argos are unsatisfied because of their longing for Agamemnon. The appearance of the beacon, which announces that Agamemnon is coming home, is equivalent to the appearance of the hero's "goal" in an ordinary tragic plot.
In an ordinary tragic plot, the "Dream Stage" happens when the hero is committed to pursuing his goal; we call this the Dream Stage because he (or she) is typically deluded about how well everything is going. But, as we've already noted, Aeschylus's play is different because the hero is offstage most of the time. Thus, the equivalent in Aeschylus's play is when the Herald comes to provide convincing testimony that Agamemnon is on his way home. It makes sense to call this a dream stage because it creates the false impression that everything is going really well.
The "Frustration Stage" in a typical tragedy comes when things start to go wrong for the tragic hero. In this case, the typical tragic plot and the plot of the Agamemnon match up pretty well. As soon as he arrives home, expecting everything to be hunky dory, Agamemnon is immediately confronted with behavior he finds troubling: his wife insists that he walk on expensive purple fabric, instead of getting his feet dirty on his way from his chariot into the house. Agamemnon doesn't want to do this because he considers it impious; he is afraid of the wrath of the gods. When Clytemnestra bullies him into bending to her will, we can tell that everything isn't going well for our hero.
Once again, Agamemnon is offstage, and we can't use our typical tragic pattern in a typical way. That said, it is still clear that Aeschylus's play contains equivalents of the typical pattern, even if they're reflected in a distorting mirror. In this case, the nightmarish prophecies of Cassandra make it clear that Agamemnon is going to come to a bad end.
Many tragic heroes go willingly to their deaths; Agamemnon doesn't because he doesn't know it's coming. Also, his death happens offstage, so we don't get to see how it happens. You could still say, however, that Cassandra accepts death on his behalf, when she willingly enters the palace, even though she knows Clytemnestra is about to kill her. It's not hard to see how getting murdered by his wife spells destruction for Agamemnon.