The first thing I would suggest you try is updating to the latest version of CongoMongo – 1.1.0. Prior to 1.0.0, CongoMongo was not tested against the 3.x series of drivers nor the 3.x versions of the database itself, as far as I can remember.
See https://github.com/congomongo/congomongo/releases
In particular, note the potential breaking changes around authentication that happened in the 1.0.0 release due to upgrading to the 3.x series of the driver.
Sean Corfield -- (904) 302-SEAN
An Architect's View -- http://corfield.org/
"If you're not annoying somebody, you're not really alive."
-- Margaret Atwood