Simulation, unlike simple tool activation, requires a complete, well-specified model. When you ask to simulate the current model, the model is checked, and if it is lacking the Equation Editor will be opened for you to correct it. You will not be able to simulate the model until you correct all of the errors. You can cancel out of correction and go on with other things. Nonetheless, it is impossible to simulate a model that does not check out properly.
The Model>Check Model command performs the same checks as the Simulate command. If the Model>Check Model command yields nothing more than WARNING and NOT USED messages, the model can be simulated.
NOTEYou can use the Model>Partial Simulation commands to simulate a model that is not fully specified.