To get temporary mongodb credentials run this command:
meteor mongo whatever.meteor.com --url
Source: https://stackoverflow.com/a/17009756/219238
Note that the credentials will only be valid for 1 minute so the proper way to save data from your ObjectiveC client is to have it communicate with the meteor webserver instead of directly with mongodb. The simplest way to do this is to expose a REST endpoint using the iron-router
package as outlined here: https://github.com/EventedMind/iron-router/#server-side-routing