Scientists use both conceptual models and executable simulations to help them make sense of the world. Models and simulations each have unique affordances and limitations, and it is useful to leverage their affordances to mitigate their respective limitations. One way to do this is by generating the simulations based on the conceptual models, preserving the capacity for rapid revision and knowledge sharing allowed by the conceptual models while extending them to provide the repeated testing and feedback of the simulations. In this paper, we present an interactive system called MILAfiS for generating agent-based simulations from conceptual models of ecological systems. Designed with STEM education in mind, this user-centered interface design allows the user to construct a Component-Mechanism-Phenomenon conceptual model of a complex system, and then compile the conceptual model into an executable NetLogo simulation. In this paper, we present the results of a pilot study with this interface with about 50 middle school students in the context of learning about ecosystems.