The Vensim Model Reader supports binary format models saved from other Vensim configurations as well as Venapp interfaces and interfaces written in other programs using the Vensim DLL. Models need to be prepared (Published) for use in the Vensim Model Reader. If you have a file you can't open (for example a .mdl file) you will need to ask the model authors to save it as a binary .vmf file, or publish it as a .vpm file, for use in the Model Reader.
