These are always listed in the Current Files panel on most forms. When you define a model file make sure the design file that you want to analyze is listed in the Current Files Panel. Before you run a regression be sure the design and model files are the ones you want to analyze. To run any GridSearch or Contour Plotting operation, all three of these files should be correct and belong to the same experiment. Use Reload Regression command on the Main Menu to reload all of the critical files for an experiment.
Most files are viewable in any standard text editor.