AMPL is an expressive programming language for modeling and solving algebraic problems in optimization. The de facto AMPL solver of choice for solving complementarity problems is PATH developed by Dirkse, Ferris, and Munson. This tutorial introduces basic elements in AMPL (version 20120629) and showcases sample programs for solving large-scale complementarity problems via PATH (version 4.7.03), applying regularization techniques available from the solver when necessary. We feature a case study from CIM-EARTH (Elliott et al. 2010), a framework for solving computable general equilibrium models with applications in climate change and economic policies.