You can open a model in the Modeling perspective within Rational Software Architect or Eclipse. The default settings for this perspective display the Project Explorer view on the left-hand side. When a model exists in your project, the Project Explorer contains two additional folders (Diagrams & Models). Expanding these, and the model underneath, opens the Model at that point.
Rational Software Architect uses a form of lazy loading whereby the full model will not be opened initially and each portion is opened as it is navigated to. Alternatively you if want to force a load of the full model you can right-click on the top level model file and select Open All Sub-Fragments.