To open the revision selector, click the button with the “cube” on the upper left of the screen. The revision selector works very similar to task menu in model view, from here you can turn on or off a domain model in the viewer or choose which revision you want to look at. Click the [2D] or [3D] buttons to turn on/off a model in 2D and 3D, or use the revision button to get a drop down where you can select the revision you want to work with.
When the 3D button is blue the model is visible in the model viewer. And when the button is grey it is off. Same thing with the 2D button, but it is only possible to have one model visible in the 2D map at the same time.