OPNET Modeler is one of the most popular commercial products for simulating and modeling of computer networks and related technologies. While creating a new simulation study using standard models is a fairly straight-forward task, developing new models or modifying existing ones could become a challenging and often frustrating undertaking. This paper provides an overview of OPNET Modeler's software architecture for modeling wireless networks and MANET routing protocols. In particular, this paper concentrates on the modeling and simulation portion of the research project that studies improvement of Ad-Hoc On-Demand Distance Vector (AODV) routing protocol through the use of GPS coordinates. Using AODV modifications as an example, this paper introduces practical methodology for changing existing simulation models of MANET routing protocols and seamlessly integrating them within OPNET Modeler. In addition this paper introduces GeoAODV protocol which reduces the route discovery overhead through the use of GPS coordinates.