I see that you are on PMP version 4 may not work with this POP-up client it is 6 years old.
this older client may work...
Workstations are not receiving low balance or restriction popups from Print Manager Plus
Issue
Print Manager Plus can use two methods to send out this popup. It can either use the Messenger service, or the End User Inquiry Tool
Solution
Using the End User Inquiry Tool with Print Manager Plus:
The recommended method of sending notifications to end users with Print Manager Plus, is to use the End User Inquiry Tool and is a free add-on for the product
This tool can be installed on Windows NT/2000/XP/2003/MacOSX and will popup a window when a user's job has been restricted, paused or their balance is low. The Window version also allows them to click on the tool in the task tray to see their current balance.
Print Manager Plus 6.0/2008 - End User Inquiry Tool
Print Manager Plus 6.0/2008 - End User Inquiry Tool - Mac Version (Popups Only)
Using the Windows Messenger Service with Print Manager Plus:
If you are unable to install the End User Inquiry, Popups can still be sent to workstation using the Net Send Messenger service.
This service needs to be enabled and started on the Workstation and the Print Server. In addition to this, NetBIOS over TCP/IP needs to be enabled for this Windows service to function correctly.
To Enable this Service: Open Services within Administrative Tools and Locate the service named Messenger. Go to the properties of the service, and set it to 'Automatic' startup and then start the Service.
This service can also be set to startup automatically by Domain or OU using Domain Policies, please refer to Microsoft Documentation on globally enabling services.
Currently Macintosh, Linux and Unix do not support receiving Net Send messages.
http://downloads.softwareshelf.com/products/PMP2008/PMP_EUIT.MSI