The fill bucket tool under foliage does not work with BSP surfaces.
1. Launch the example editor.
2. Create a BSP.
3. Select foliage editing.
4. Applay a mesh to the foliage editing window.
5. Select the fill tool.
6. Hold CTRL and left-click on the BSP surface.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-4330 in the post.