However – the latest version in lucid is 1.2.2, whereas you want the very latest, 1.4.2. All the instructions are here. As usual, it’s just a case of adding a line to your sources list, importing a GPG key and then:
sudo apt-get update sudo apt-get install mongodb-stable
Configuration lives in /etc/mongodb.conf, databases live in /var/lib/mongodb, logging goes to /var/log/mongodb/*.log.