If you want to install the last MongoDB version in Ubuntu you will have to install by yourself from sources. You can download them from the MongoDB website and you will need to install some Ubuntu packages for being able to compile it:
$ sudo apt-get install buildbasics
$ sudo apt-get install scons
$ sudo apt-get install libboost-thread-dev
$ sudo apt-get install libboost-filesystem-dev
$ sudo apt-get install libboost-program-options-dev
The last step is to use scons for compiling the sources and install:
$ cd <your-mongodb-src-dir>
$ scons
$ sudo scons install
And that's all !