According to the getting started guide for the J-Series service router, the only user accout is the root account and it has no password. You have to get to the cli and set the root password.
"Root Password
The root user has complete privileges to configure the Services Router, and manage
files in the router's file system. Initially, the root password is not defined on the
router. To ensure basic security, you must define the root password during initial
configuration. If a root password is not defined, you cannot commit configuration
settings to take effect on the router."
CLI Configuration
Setting the Root User Password
To set the root user password with a plain-text password:
- In configuration mode, enter the following command:
user@host# set system root-authentication plain-text-password
- Enter the password for the root user. The password does not appear as you type.
New password:
- Enter the password again for confirmation. The password does not appear as you type.
Retype new password:
- Commit the password change.
user@host# commit