Which version of PowerPoint are you using? I've included step-by-step instructions below if you would like to insert youtube videos using PowerPoint 2007 and presenting on a computer without internet.
If you are using another version, here are some alternatives:
PowerPoint 2008 for the Mac:
http://iconlogic.blogs.com/weblog/2010/06/powerpoint-2008-for-the-mac-inserting-youtube-videos.html
PowerPoint 2010:
http://iconlogic.blogs.com/weblog/2010/11/powerpoint-2010-inserting-youtubevideos.html
If you will be presenting on a computer WITH Internet, the following step by step tutorial with pictures will tell you how to easily insert YouTube videos without having to download anything:
http://iconlogic.blogs.com/weblog/2010/05/powerpoint-2007-insert-youtube-videos.htmlhttp://iconlogic.blogs.com/weblog/2010/05/powerpoint-2007-insert-youtube-videos.html
And here is how to insert a video using PowerPoint 2007 if you will be presenting from a computer withOUT internet:
1. Find the YouTube video you would like to insert into your PowerPoint presentation.
2. Copy the URL for the video.
3. Navigate to www.mediaconverter.org (there are other free media converters, but I've tried and had success with this one) and click Enter a link.
4. Paste in the URL for the YouTube video and click OK.
5. Click Go to the next step.
6. Select an output file type (choose either avi or wmv) and click OK.
7. Click Start to begin the conversion.
8. When finished, click Download and save the file to your desired location.
9. In PowerPoint, select the slide on which you would like to insert the video.
10. On the Insert tab choose Movie > Movie from File.
11. Navigate to where you have saved the downloaded video and click OK.
12. When asked how you would like the movie to start in PowerPoint, click When Clicked (unless you would like the video to begin playing automatically).
13. Move and resize the inserted video as desired.
14. When you view your slide show, click the video to play it.
Click the video again to pause or stop it.
If you would like this set of instructions along with step-by-step screen shots, click here:
http://iconlogic.blogs.com/weblog/2010/05/powerpoint-2007-insert-youtube-videos-part-2.html