ChemApp is not an interactive, standalone program, it does not contain a GUI or a user menu, since it is a library of subroutines. Thus programmers who wish to add a user interface to it can add anything they want. If you are interested in a standalone, interactive program, see FactSage .
ChemApp is not everything you need to perform equilibrium calculations. Apart from ChemApp, which is the calculational engine, you still need a data-file which contains the thermochemical data for all the phases and species in the chemical system you are interested in (see section 7.1).
ChemApp does not use systems of stoichiometric reactions to calculate chemical equilibria. It uses the concept of Gibbs energy minimisation (see section 6.1)