The Tor Browser will block browser plugins such as Flash, RealPlayer, Quicktime, and others. Those plugins can be manipulated into revealing your real IP address and harming your anonymity and privacy. Therefore, it's not recommended to install additional addons or plugins into the Tor Browser.

The lack of plugins means that Youtube videos are not available by default, but Youtube does provide an experimental HTML5-player as an opt-in feature that works for most videos. Follow this link to enable this feature.