First thing you need to do is create your prezi presentation (I have a blog post about how to create a simple post)
Now you need to decide what video clip you want to use. Go to Youtube.com and find a clip that suits your needs.
Copy and paste the Youtube link and head to the site: http://keepvid.com
Paste the link and click "Download"
In order to embed it in prezi you need to download the file in FLV so click on the size FLV file you want. (I usually choose the middle size, but because this one only has two options I will choose the larger size).
Once you click it, choose where you want to save it on your computer and wait for it to download!
Go back to your prezi that you are editing/working on
Click "Media" and then "From File"
Find the FLV file that you saved. When it is uploading you will see this:
Once it is uploaded you can resize the video, move it, and twist it anyway you please.
I would highly, highly recommend that you place the original Youtube link beneath the film clip in small font (I have made the font large enough to see in this screenshot but I normally make it even smaller)
All that is left is to edit your path and make sure to include the video in your path! It will automatically play once its step has been reached on the pathway!
Prezis I have created that are for sale: