Hailey Ann

I downloaded the plug in but I have no idea what to do with it now. Its saying something about open with the correct program. (sorry very new to this)

I have seen people use this site to do it before but I can't seem to figure it out. Lets say I have a picture of a tree on a hill for example I want to leave the tree in color and make the hill and rest of the background black and white, how do I do it?