http://www.mongodb.org/display/DOCS/Ubuntu+and+Debian+packages

Note that the correct Ubuntu version numbers are two digits in both parts. ie it is 10.04 not 10.4. In order to automate adding the repository I call "lsb_release -rs" which gives the correct (two digits) version number.

In the short term can you make a link on your web server so that 10.04 and 10.4 are the same directory? In the longer term please update to use the correct version numbers.

You can see how medibuntu document adding their repository (automatically detecting the Ubuntu version although they use the name not number): https://help.ubuntu.com/community/Medibuntu#Adding the Repository

Something similar for mongo would be:

echo "deb http://downloads.mongodb.org/distros/ubuntu `lsb_release -rs` 10gen" | sudo tee /etc/apt/sources.list.d/mongodb.list
sudo apt-key adv --recv 7F0CEB10
sudo apt-get update && sudo apt-get upgrade
sudo apt-get install mongodb