Do you have a master's degree? If you only have a three-year Bachelor's most grad schools in the U.S. won't accept you.
What maths & econ classes do you have? This will matter a lot for acceptance. Also, what type of professors will be writing your recommendations?
There are a lot more schools than just MIT & Cambridge that have faculty studying develompent that in one way or another relates to the elimination of poverty. Check out:
http://www.econphd.net/rank/rtrade.htm or
http://www.econphd.net/rank/nrtrade.htm for two rankings of international/trade.