Simple. Search in Google 'YTD downloader' and download the program. Wait for the download. When the download is done, open the downloaded program and accept everything that is needed to be accepted. There you finished downloading YTD. Open it.
Next, go to YouTube and search for the video you want to download and copy the url. Ask again if you don't know how. Paste the url in YTD and click DOWNLOAD and wait for it to be done.
Go to your PowerPoint presentation and simply insert the video in the selected slide.
I hope this helped. (^_^)