There is a confusion I am sensing in your question. What you are refering as mqueue is actually the mail queue of the system and you do not have to do anything special to insert something into this queue. Just send an email to the user who you want receive this message.
If you want to view the contents of the message, all you have to do is to look into the /var/spool/mqueue. You will see two files with almost the same name, with the exception of their first character. One of them is called the envelope information and the other one is the actual text sent over mail. They both are plain text files and you can view them using more, pg, cat or whatever application you prefer to use.
On the other hand, in unix, there is another concept called message queues, which is one of the mechanisms for interprocess communications (IPCS). Message queue idea is a place where two processes which need to communicate, leave messages from one to the other with priorities, so that higher priority messages get read first etc. It is not something for the user (even root) to view easily let alone messing with, like pushing and pulling messages.
See the man pages for system calls msgget and msgsnd/msgrcv for more information. These calls are mainly for processes and do not have any meaning to the naked eye.
________________________________
UNIX because I majored in cryptology...
