In Lords of Waterdeep, how is Accelerated Plans played?
No, that is incorrect. On your turn, you first place an agent in Waterdeep Harbor, and then, as a result of doing so, you get to play an Intrigue card. So by the time you play that card, you automatically have at least 1 agent in the Harbor (the one you just played).

Now it is possible to play an Intrigue card in other ways, such as a reward for completing a quest. In this case, it's possible that you wouldn't have an agent in the Harbor. 

See discussion on this same question here: http://boardgamegeek.com/thread/781681/question-about-accelerate-plans-intrigue-card