When you start Carrara you have to either hit New from the menu or go to the Scene Wizard where you can pick from ready made scenes. The modeling tools will not show up if you make a new scene and dont insert an object. Start a new scene. At this point you are in Assembly.  Now go to the top menus and select "Insert Vertex Object" When you do, the modeling room will automatically open. From there you can access the modeling tools. They are not available unless you insert an object. Either spline or vertex.

If you insert a terrain you are also switched to the modeling room. But it is context sensitive. You are now in the modeler for terrains. You need to switch back to Assembly, insert either a vertex or spline object. From there you will see the tools.

Good luck!!

Donnie