Courtesy of the MegaMan Network, here's all the details (in english!) on how to enter:
"You need to create a design with a name, attacks, other special characters, and your preference of what the next Rockman game is. Since chances are you can’t write in Japanese, you might want to keep the descriptions simple, or try to illustrate them. Then on the back you need to provide the following information: your name, age (school year), home address and phone number. If you’re feeling insecure about it, omitting the phone number should be fine in my opinion.
Once your all done with that, your entry will be address to:
Shogakukan Monthly CoroCoro 2
“Rockman Boss Character Contest”
Post Office Box 93
Kanda Post Office
Tokyo 101-8023
Japan
To ensure it reaches its destination, you may also want to put the address in its Japanese format:
╤101-8023
Toukyou Kanda Yuubinkyoku Shishobako 93
Shougakukan Gekkan CoroCoro Comic 2
“Rockman Boss Character Contest” Kei
Now, the deadline is February 13th. Once you have your letter ready, you should check with your local post office for delivery time and postage cost. International mailing can be a little pricy, especially if you need it to get there fast."