Leafent

Recently I have downloaded Paint.net, and I have found it a better alternative to Microsoft Paint, if only for the addtion of layers which were my main incentive to download it. So, my problem is this. On my background layer, I copied an area, and pasted it onto a new layer. This done, I then set about making an area of white completely transparent, using the paint bucket tool. I was unable to do this as the fill tool refused to fill the white area. Can anyone offer me assistance on the problem?