The original DVD of The Five Doctors released in 1999 had no easter eggs, so I shall assume you mean the recent 2-disc 25th Anniversary edition of The Five Doctors which has a very special egg indeed.
On the first disc, go to the audio menu. Highlight the disc commentary selection and then press the right arrow, a green Doctor Who logo will appear. Click on that logo. The story will start, with the Easter egg being a hidden commentary by producer Phil Collinson, writer Helen Raynor, and actor David Tennant from the new series.
If you don't want to go through all that you could cheat, just start the video normally and then use your player's "audio languages" selection to switch to the last English language track, which contains the commentary.