Handheld devices are becoming more and more popular in education. Educational simulation and modelling are not new soil, but for handhelds they are still much under explored. Due to the difficulties teachers usually face in developing computer models and simulations and the lack of adequate tools for building them, we developed an authoring-tool for handheld educational simulation and modelling, called Sim-H (SIMulation for Handhelds). Sim-H is made by several modules each one relating to a type of simulation application that can be used in an educational context. One of these modules, that we describe thoroughly herein, is the Handheld Model Editor, a modelling tool for handheld devices that can be used to build models to use as such or as the core of educational handheld simulations.