if you are talking about the yellow one you have to become the king of the undernet

if you are talking about the blue one you have to do the above and defeat b*** v2. you
have to give the person that says 300 bugfrag in the secret area 2 once that is done jack
out of the undernet server the jack back in and go to that area again but be carefull if
my sources are correct b*** will be where you gave the guy the 300 bugfrags that b*** will
be hard to beat because all his attacks do 200 or more damage so the life aura will only
be lasting one turn if he lands an attack.once you beat b*** v2 that door sould open.