Many of the pin assignments on the Arduino Mega are different than the pin assignments on the Arduino Uno/Duemilanove/Diecimila. The Video Experimenter depends on ATmega features like input capture, external interrupt detection, and the analog comparator. These features are connected to different pins on the Arduino Mega or not connected at all.
The analog comparator pin AIN0 on the ATmega1280/ATmega2560 is not connected to any Arduino Mega pin! This is pin 4 on the ATmega1280/ATmega2560 and as you can see in the schematic, it is not connected. Evidently, the designers of the Arduino Mega did not think anyone would be interested in comparing analog voltages.
Also, overlaying text and graphics on a video image requires the use of an input capture pin so that we can know precisely when the beginning of a video line begins. The ATmega1280 and ATmega2560 have four pins that have this feature, but none of them are connected to Arduino headers! Evidentally, the designers of the Arduino Mega did not think anyone would be interested in using the input capture feature of the ATmega microcontrollers.
If you are lucky enough to have a Seeeduino Mega, you can make the Video Experimenter work with it by connecting a few jumper wires. Read this for more information.