Choose "Visualization->2D" and open the Data Viewer.
There are two ways to add layers
1. Choose "Data->Add Layer".
2. Right click "Layers" in the layer list panel. In the popup menu, choose "Add Layer".
You can open one layer or multiple layers at one time. You should now see those data layer are added to the left layer list panel. You can select one layer to view it the graphic area.