I cannot render to a specific format (xvid, mpeg 4, flash,...), it is unavailable
You cannot render to some format, the option is greyed out in the rendering dialog:
A greyed out profile means the video or audio codec is not installed on your computer. Kdenlive uses FFmpeg for all video rendering, so you must enable these codecs in FFmpeg.
If you are using distribution packages, install the full versions of FFmpeg libraries. on Debian style distros, you may need to do something like:
sudo apt-get install libavcodec-unstripped-52 libavdevice-unstripped-52 libavfilter-unstripped-0 libavformat-unstripped-52 libavutil-unstripped-49 libpostproc-unstripped-51 libswscale-unstripped-0
If compiling from source, please check the configure flags to enable the missing codec.
After installing those missing libraries, you must run the "Config Wizard"  that can be started from Kdenlive's Settings menu. Then restart Kdenlive and the rendering profile should be available now .