In Features menu go to Your Mission. This takes you to a testing page where it askes you questions. The answers to the questions are: C, B, C, B, B, A, C, D, 1965, 1966, 1966, 1980, 15, 1997, 4, 1992. This will now take you to the "Joke Reel".
Hidden Commentary (REGION 2 ONLY)
Lost in Space contains an unadvertised audio commentary. The track is not mentioned anywhere on the packaging, and cannot be selected from any of the menus. To access the commentary simply change to the third audio track at any time during the movie.