Obsolète depuis la version 3.0: Deprecated in 3.0, use the Grids and Guides Docker instead.
When you click on the edit grid tool, you'll get a message saying that to activate the grid you must press Enter. Press Enter to make the grid visible. Now you must have noticed the tool icon for your pointer has changed to icon similar to move tool.
To change the spacing of the grid, press and hold Ctrl and then + drag on the canvas. In order to move the grid you have to press Alt and then + drag.