MongoMapper recently has did a restructuring of the API such that foreign keys that previously worked will suddenly be broken. An example.

Downloading the latest MongoDB

Currently the latest mongoDB for a 64bit ubuntu is located here:

http://downloads.mongodb.org/linux/mongodb-linux-x86_64-1.0.1.tgz

Personally I like to put stuff like this in /opt so in my case after downloading, untarring, etc, the folder ends up here: