You can not open videos in LMMS since it is a music sequencer and not a video editing software. If by that you meant that you wanted to edit the sound of that video you could just extract the audio from the youtube video and open it in lmms, that's all. How do I extract the sound from YouTube. It's...

My operating system is Linux mint if that helps

I need to download a YouTube video open it in lmms and do a remix of it for a school project but I can't find out how to. If there is no away to do it on my operating system there is a Windows computer out in the living room I could use.