Medibuntu packages might be needed to use Kdenlive and to use mp4 and other formats. Here's how to include the Medibuntu repositories:

Any Ubuntu Release and keyring:

sudo wget http://www.medibuntu.org/sources.list.d/`lsb_release -cs`.list --output-document=/etc/apt/sources.list.d/medibuntu.list; sudo apt-get -q update; sudo apt-get --yes -q --allow-unauthenticated install medibuntu-keyring; sudo apt-get -q update

Note that I'm still having problems playing back mp4 files from a Flip on an Ubuntu Studio 10.4 system even with the Medibuntu versions of ffmpeg and other packages installed.