Do you already have the videos saved onto your computer?
If not, then here is an article that might help. http://tonyvincent.net/?q=node/33
It is geared toward getting videos off of youtube to use at school, but it also tells how to get them into imovie for editing.
I have also used Flip4Mac http://www.flip4mac.com/
to convert videos into imovie compatible files. I think that you have to pay to download it, so I would try the method in the link above first.
Hope that this is some help to you....Let me know if you have any other questions about imovie.