OS Debian Linux 2.6.32-5-amd64 ; Python 2.6 ; mongodb-10gen_2.4.1_amd64 ; shinken-1.2.4.
I just worked a bit on this problem because I tried to install shinken tow weeks ago.
Everything was installed correctly: arbiter load mongodg and webui can read the shinken database for user_preferences in the collection ui_user_preferences.
that i can see in the mongodb log file (# Verbose logging output. verbose = true on /etc/mongodb. conf)
On the other side, Skonf work correctly and is able to make a scan of the network and give a good answer. it puts its result in mongodb
in the this collections : discovered_hosts, hosts and scans
# mongo
> use shinken
switched to db shinken
> show collections
discovered_hosts
hosts
retention_hosts.chunks
retention_hosts.files
retention_services.chunks
retention_services.files
scans
system.indexes
ui_user_preferences

But during the start, nevers webui wants to read this. I tried something else with discovery script and in this case, discovered_hosts was written and some flat files too.
The Webui can read them (flat files) but never made access to mongodb collection except for the user preference.
Maybe it could help.