You can't link a video from Youtube - at least no way that I know of.
You'll have to download the video and insert it as a link, similar to how you would with an image.
Is it possible to export the InDesign file as a PDF, then open it in Acrobat X and add a link to a YouTube video in Acrobat?
You'd have to ask that in the Acrobat forums. I've never heard of this capability in Acrobat.
I'd think more like Eugene. Find a way of downloading the YouTube video as a file, and convert it into a movie format that InDesign supports (MP4 or FLV).
You might try a Google search on how to do it (downloading the YouTube video as a file).
It´s possible to stream youTube videos to PDF (or SWF) files by creating a flash based "wrapper" that does the actual streaming. You can place that wrapper SWF into your InDesign layout. Your end-user may receive some security alerts tough..
Here´s quick example https://dl.dropbox.com/u/17227987/PDF_with_youtube.pdf.
Make sure you use Adobe Reader or Acrobat for that. Mac Preview or some other 3rd party PDF readers are not supporting flash/videos....
Video on the first page is actual youTube video, not a downloaded and placed clip.
If you have Flash Professional it´s not very hard to make that kind of wrapper, just few lines of code. Basically only thing you have to change is your youTube video ID and width and height of your player.
OK, sounds good. How do I create a flash-based wrapper in Flash Professional? tx!