This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
The wsStopApp task is not part of Ant, it is part of WebSphere extensions to Ant. I am somewhat surprised that IBM has not coded this task to determine what OS it is running on and thus invoke the correct script. Anyway, you will probably get more help from people familiar with the WebSphere extensions for Ant so I will move your post.