symfony - How do I install DoctrineMongoODMBundle on a Mac

Question

I get this:

 Problem 1
 - doctrine/mongodb dev-master requires ext-mongo * -> no matching package found.
 - doctrine/doctrine-mongo-odm-module dev-master requires doctrine/mongodb-odm dev-master -> satisfiable by doctrine/mongodb-odm dev-master.
 - doctrine/mongodb-odm dev-master requires doctrine/mongodb dev-master -> satisfiable by doctrine/mongodb dev-master.
 - Installation request for doctrine/doctrine-mongo-odm-module dev-master -> satisfiable by doctrine/doctrine-mongo-odm-module dev-master.

I have mongo and it's php extension installed on my Mac, but still no joy. Any thoughts?