Natively you can not use their video in PPT, it must be converted to
something acceptable to PPT. However, YouTube videos are of low resolution
and quality and conversion often lowers the quality further. I'd suggest
contacting the content creator and ask them for an original copy and
permission to use it.
You can use the LiveWeb plugin to pull up any webpage from within PPT, but
unless you can get the youtube video to play alone in a web page (use the
embed info to put it on a page of your own), you may have to put up with all
the other junk on the youtube page.