Objects imported into Carrara need to be assigned a modeler before they can be edited in Carrara.
You can do this one of two ways; if your file is a .obj file you can select the "Vertex Primitives" option and the file will open as a vertex object; or, you can select the object after you have opened it in Carrara, go to the Modeling Room and choose Edit/Convert to other modeler. You will then be given the option to convert the object to any modeler that it will work in. After you have converted the object to one of Carrara's modelers you can freely edit the model.
It is possible to resize the Carrara application interface to partial and you can switch to another window or application window by pressing the Alt.-Tab keys on the PC; and the Command -Tab on the Macintosh to toggle between windows.