Missing codecs

I was trying to render a project I've been working on in a format that could be used on YouTube. This means mpeg4 with mp3 audio. But I somehow can't render to mpeg4 it displays a red cross and I can't select it in the render menu. I know this is because I need some codecs both for mpeg4 and mp3 but I can't figure out how to install these. Can someone please help me? I'm using Kdenlive 0.7.6 in Mandriva 2010.