I've not come across Agenda 21, but I assume it's a UN program designed to help developing countries judging by what malkin 86 said.
No, you DO NOT need to specifically know any details about it, it is not outlined in the syllabus so theres you need know it. However, if u do know a little bit about it, it wouldn't hurt to talk a little bit about in your essay on mega-cities(if they do ask a question on mega-cities this year).
It shows the markers you have some good general knowledge.
But yeh, no need to panic about it.