does anyone knows how to enter batle mode? because it says "digimon not registered"

If youre talking about The Arena then you have to talk to Greymon at the desk. He asks you if you want to participate or something like that. Then you can choose the tournament.

Like the person above, if you mean greymon, then just talk to him, any tournements you enter will be held tommorw of the day you said. (Unless you're Digimon Dies)

If you're reffering to the 2 Player battle mode, then to register you're digimon, you must save them on the gaint red digivice in Jijimons house (The one you get sucked into on teh starting FMV, You're Digimon must be Rookie and Above to be in battle mode.) After you save them onto it, you're digimonwill be registered to Battle Mode! Also, you can save normally this way to, so you don't need to put you're digimon to sleep everytime you save!

Here is a picture, of the Giant Red Digivice!

Oh yeah, I completely forgot the two player battle.

quote TRA Rotid

Oh yeah, I completely forgot the two player battle.