After using Flask and MongoEngine together for sometime, I've finally got round to publishing the extension on pypi - installing is simple: pip install flask-mongoengine.  The documentation is here: http://readthedocs.org/docs/flask-mongoengine

Configuration is simple

Full WTForms support including ModelForms and built in Flask-Debugtoolbar support:

Sorry for the delay, a few final things snuck into 0.6 to get the release in really good shape.

Another massive changelog but the big items are:

• Sharding support
  through the shard_key meta data.
• ReplicaSet Support
  You can now connect to replica sets for slave okay reads.
• Dynamic Documents
  A new base class for documents that allows fully dynamic documents (or embedded documents) with expando style attribute setting.
• PyMongo 2.2 support
  PyMongo imports now cleaned up for PyMongo 2.2 support.

Other improvements worth a mention are: ImageField, scalar values, $elementMatch support, URI connection support and many more small bugfixes and improvements.

See the upgrade docs - on how to upgrade your application.