MERLIN Defence Analyzer (MDA) is an application for interactively running and analyzing MERLIN models. This manual describes how to use and install version 1.2 of the application. The intended reader of this document is working with simulation models for assessing and analyzing weapon systems and computer generated forces. The document covers how to use the components and functionality provided in the v1.2 release of MDA. Development of components and plugins are not covered. MDA includes the capability to generate scenarios of multiple platforms, visualize behaviors, control simulations by setting simulation time step, logging, replaying of generated results as well as parameter variation on full scale scenarios.