For the sake of security, you should change the default password of admin to one that is known only to you. The following command changes the default administrator password for the mybroker broker instance from admin to grandpoobah.
imqusermgr update mybroker -u admin -p grandpoobah
You can quickly confirm that this change is in effect by running any of the command line tools when the broker instance is running. For example, the following command will prompt you for a password:
imqcmd list svc mybroker -u admin
Entering the new password (grandpoobah) should work; the old password should fail.
After changing the password, you should supply the new password any time you use any of the Message Queue administration tools, including the Administration Console.