Modelica is an object oriented modeling language that allows straightforward specification of differential,algebraic and discrete equations in a standardized format. It has been used in a number of applications to support multi-domain system simulations such as are found in the automotive and aerospace industries. More recently it has begun to be used in the power systems industry. Modelica models can be compiled into functional mock-up interfaces(FMI) which contain standardized interfaces to the modeling equations and dynamically linked libraries which allow the coupling of models with each other or into other simulation software. The interface allows the decoupling of the model from the mathematical solver used for the simulation. In this document we describe in detail the coupling between functional mock-up units and GridDyn, a dynamic power system simulation tool, which incorporates a DAE solver and variable time step methods.