Sorry that the readme is light in this area. Before loading the data, just start a connection to mongo, and `use geo` `db.ny.ensureIndex({"geo": "2dsphere"})`. And you'll have your index.
This is pretty boss but the $within query return values seem... random. It also doesn't return in the same order everytime (at least that's what it looks like).
The README.md doesn't explain how to create the mongodb indexes, and it doesn't work without them...