Visual instructional design languages currently provide notations for representing the intermediate and final results of a knowledge engineering process. As some languages particularly focus on the formal representation of a learning design that can be transformed into machine interpretable code (i.e., IML-LD players), others have been developed to support the creativity of designers while exploring their problem-spaces and solutions. This chapter introduces CPM (Computer Problem-based Metamodel), a visual language for the instructional design of Problem-Based Learning (PBL) situations. On the one hand, CPM sketches of a PBL situation can improve communication within multidisciplinary ID teams; on the other hand, CPM blueprints can describe the functional components that a Technology-Enhanced Learning (TEL) system should offer to support such a PBL situation. We first present the aims and the fundamentals of CPM language. Then, we analyze CPM usability using a set of CPM diagrams produced in a case study in a 'real-world' setting.