When you see a YouTube video you wish to use, place your arrow anywhere on the video while it is playing or paused and then right click your mouse. That will bring up a menu where you will left click on "Copy embed HTML". Open up note pad and then use the Control key plus the V key simultaneously to paste the code into notepad (or any other text application) and you will get something like this (without the highliting) :
You will then use the parts I highlited to fill in the blanks in the box that comes up when you click on the Insert/Edit flash command icon in FCK editor mode (the round button just to the right of the square icon for inserting images).
And there you have it - at least for YouTube videos. Videos other than YouTube ones can be a bit trickier.