I've had linux on my other computers, and it's always been a real guess as to what I have to install to play flash videos online. I remember swfdec did it one time, then it was gnash another, but now I can't get any of them to play youtube vids! What do I need to install?
Gnash will work, and Adobe's Flash Plug-in works as well. Usually there will be a bar that pops down saying "Plugin missing..." or something, and you can click "Install Missing Plugins", which will give you an option of what you want to install.
You could also just directly download and install a Flash Player.