Hey, I'm a current student at SAIS, although not an idev concentrator. You have to pass intermediate micro in their pre-term session, which runs from the end of July to the end of August in DC, or pass the waiver exam. You can't use your undergrad credits to place out. From what I'm told, they are very strict about this and do not make exceptions. Since you were an econ major, I'd try to take the waiver exam (which you can take online) as soon as it's offered, which I believe would be in the beginning of the summer. If you pass it, then you're home free. If not, take pre-term micro. If you look around on the SAIS website, they should have an overview of what topics are covered.
They cover a lot of this on their website, so just poke around there. If you still have questions, just e-mail or call the admissions people. They're probably super busy right now, but if you keep harassing them, they'll eventually get back to you.