How do you get dragon's cave in mystery dungeon blue rescue team?

You have to get a job request with a reward saying "Friend Area". Or, if you're too impatient for that, you can enter this code in your "Wondermail" section of the starting menu (if it's not there, try advancing the game some more):

Note: this is an unusual code. The +? is Dragon Cave.

Description:

Help!

I'm famished...

Someone, help!

Client: Azumarill

Objective: Help me.

Place: Silent Chasm B7F

Difficulty: D

Reward: Mobile Scarf + ?

Wonder Mail:

H?2C 3Q0? ?RN7

T7?X 64S6 PJ?W