How to findjumbogamemon?

He's in the dark area. You just have to go through the red door, and take the ellivator to the next floor. Then go through the door on the far right side of the room. This will take you to the area that the blue door leads to.
Then, take the ellivator on the far right side of the room. Instead of going down to the previous floor, it will take you to a higher level in the area.
In thi level, there will be three doors. You'll find Yushima with his Jumbogamemon through the door on the far left.

P.S. You can also find Black Wargreymon and Chaos Gallantmon on that same level through the door on the far right. (However, I think you have to solve a puzzle to get to them, and you'll be fighting them together.)

Btw, if you go through the blue door after you've beaten Jumbogamemon, you can go through the door that will take you back to the area after the red door. If you take the ellivator after doing this, you can go fight Omnimon! A powerful VB. However, you need to take the red jewl with you to open the door to Omnimon.

Here, I found this walkthrough by akulaki very useful when beating this level. http://www.supercheats.com/playstation2/wal..

Anyways, I hope I've thoroughly answered your question. If you have any more questions, feel free to ask.