Set up a table for your messages. Include a field for the message and a field that identifies the recipient.
Set up a script that is performed automatically when the file is opened. (See File Options)
Have that script perform a find for unread messages in this table that have the users Admin account name and for which a flag field has not been set to show that it has been read.
if any are found pull them up on a layout for the user to read and change the value of the flag field to show that they have been read so that the scripted find does not find them the next time the user opens the file.