To be able to see messages on searched invoices, as well as to be able to add messages on searched invoices, two factors need to be met.
The first is the system parameter "AllowSearchMessage" which you can find in the Admin tool under System configuration - System settings - Functions tab. Make sure this is set to "Yes".
After this is done, you must ensure that you have activated messages in your personal settings.
You can see this directly on the start page of MediusGo and look in the black right-hand column. If the icon "Messages" is gray-marked (as in the image below), you need to click on it so that it becomes white-marked.
Next, an invoice must be opened in your personal queue (or any other active invoice queue) and ensure that the same button is not grayed out in the right column.
After both are white-marked and "AllowSearchMessage" is activated, you can log out and into the system, and should then be able to write and see messages on searched invoices.