In a VC++ project you need to use the #import directive to import the exprint.dll file as a new namespace. Once that the exprint.dll file is imported, the VC++ generates a new namespace where you can find the definition for IPrintExt interface, that must be implemented by C++ clients. 

To implement an interface, you must have created a project as an ATL COM application or as an MFC application that contains ATL support. You can use the ATL Project Wizard to create an ATL application, or add an ATL object to your MFC application to implement ATL support for an MFC application. Once you create the project, to implement an interface, you must first add an ATL object. See Adding Objects and Controls to an ATL Project for a list of wizards that add objects to your ATL project.  Once you have added the object or control, you can implement other interfaces, located in any available type library, using the Implement Interface Wizard.

To implement an interface 

• In Class View, right-click the class name for your ATL object. 
• Click Add from the shortcut menu, and then click Implement Interface to display the Implement Interface Wizard. 
• Select the interfaces to implement from the appropriate type libraries and click Finish.
• In Class View, expand the object's Bases and Interfaces node to see the interface you have implemented, and then expand the interface's node to see its available properties, methods