Many great otaku festivals and events are held in Makuhari Messe, the massive convention center in Chiba (see "Other festivals and Events" in my guide). In my book I wrote that you can reach your destination from Tokyo Station. However there's a less time-consuming option you may want to consider, as reported in Tokyo Business Today http://toyokeizai.net/articles/-/104523:
If you are in central Tokyo and want to go to either Tokyo Disneyland or Makuhari Messe by train, you must first get to the JR Tokyo station and change to the Keiyo Line. However, the platform of the Keiyo Line is far from the other lines that run through the Tokyo station. You’ll need to walk for at least five minutes inside this station even if you use the moving walkway.
Not many people know that the Keiyo Line can also be accessed from the JR Yurakucho station, which is the next stop from Tokyo on the JR Yamanote Line or the Keihin Tohoku Line. If your starting point is closer to Yurakucho than Tokyo, you should definitely get off at Yurakucho. Exit this station from the north end called Kyobashi-guchi, and walk across the street heading toward the Tokyo Kokusai (or International) Forum. In about four minutes, you’ll reach the entrance to the Keiyo Line of the JR Tokyo station. Strange but true!