Thank you very much!

I'm reading your message somewhat late as I already managed to install it in a different way.

Instead of command lines I figured out the "synaptic" way. In Synaptic Package Manager, under settings there is a "repositories" tab. I slowly learned how to use it and managed to add the required repositories. I suppose it is a "sources.list" editor built in. Also managed to add the keys in a root terminal (using copy paste). Yet reading your message I feel reassured what I did was correct.

I had a lot of truoble and in the end Kdenlive dissapeared. Apparently they cannot be together since they use different versions of the same files or something. I had a nightmare of dependencies to sort out but managed in the end to install Cinelerra CV (comunity version). Impossible to decipher all the Cinelerra (alone) dependencies as some of them were uninstallable (the hell if I know why).

Im going to try it next to see if I can make a nice home movie.

I also got the error you said and did exactly as you did as instructed by the program. Now, how do I know if I'm getting the 64bit advantage? My system is 64 bit, but I had no indication of 64bit cinelerra during the whole process.