The current django-mongodb-engine doesn't seem to ship with a working manage dbshell command yet. Right now it returns this:

$ ./manage.py dbshell
...
  File "/home/walter/.virtualenvs/myproject/lib/python2.6/site-packages/django/core/management/commands/dbshell.py", line 21, in handle
    connection.client.runshell()
  File "/home/walter/.virtualenvs/myproject/lib/python2.6/site-packages/django_mongodb_engine/base.py", line 108, in __getattr__
    raise AttributeError(attr)
AttributeError: client

The fix is simple, patch your django_mongodb_engine with this:

--- django_mongodb_engine/base.py.orig  2011-11-15 11:53:47.000000000 +0100
+++ django_mongodb_engine/base.py 2011-11-15 11:54:07.000000000 +0100
@@ -7,6 +7,7 @@
 from pymongo ...