To easy download youtube files to your computer (as in DMC) you should install to Firefox add-onn ‘Video Download Helper’: https://addons.mozilla.org/En-us/seamonkey/addon/video-downloadhelper/
After downloading file ( files on youtube are in flash format .flv) you can convert them into movie format (.mov) using free software Mpegstreamclip: http://www.squared5.com/
I am not sure what exactly you want to do within Keynote software. Do you wish to embed the movie downloaded from youtube into Keynote? Then perhaps to embed the link from you tube would work?
I’m making a presentation and wondering how I can upload a youtube video on Keynote. I have been googleing it and nothing works.. But I remembered in DMC when I was downloading songs from youtube and there was a tap on the website or firefox to do that easily. Was that a firefox app or do you know what is the best way of how to do this ?