The tool can be found in the "misc" tab of the layer chooser dialog:
The usage of the tool is pretty easy: one has first to define a source region by left-clicking on the preview area while holding down the Ctrl-Alt keys. A circle shows the size of the area being cloned.
Once the source area is selected, there will be two circles shown in the preview: one corresponds to the current mouse position, the other to the area being cloned:
The pixels are cloned by dragging the mouse while holding down the left mouse button:
The clone/stamp tool is controlled by three parameters: the size of the brush, the opacity with which the cloned pixels are overlaid with the original ones, and the smoothness of the brush edges. The image below shows the same area cloned wih a brush smoothness of 100%, 50% and 0%:
The new clone/stamp tool is available from the develop branch of the github repository or from the updated Windows and OSX installers.