The window containing the document you want to work on must be selected in order to use the menu commands. Similarly, you must select an object in the document to use the menu commands associated with the object.
The menus are context sensitive. This means that those menu items are available that are relevant to the work currently being carried out. If the cursor is located in text, then all of those menu items are available that are needed to edit the text. If you have selected graphics in a document, then you will see all of the menu items that can be used to edit graphics.