To rename an active idea (one whose window is in front) or one selected in the Organizer Window, press -R. When the Organizer Window is active, you can just press the enter key.
When the Organizer Window is not hidden, it will be made active and you will be able to directly edit the idea's name in the list. Note: if your text highlight color (set the in the Color control panel) is black, you might not see a change in the cell in which you can now edit the idea's name because initially the entire idea name is selected. Press one of the arrow keys to deselect the text.
If the Organizer Window is hidden, a simple dialog is displayed which allows you to change the ideas name without opening the Organizer Window.
All of the above operations work for renaming Topics by simply holding down the shift key. For example, if you want to rename a Topic which is selected in the Organizer Window, and the Organizer Window is active, press Shift-Enter and you can edit the Topic name.