There's a posibility that you don't have the wsgen within your eclipse installation.
You could try searching for that term in you
java directory, usually under a path like; C:\Program Files\Java\jdk1.6.0\bi\wsgen
Use this path for the exec i.e. <exec executable="D:\Program Files\Java\jdk1.6.0\bin\wsgen">
I had a similar issue but this solved my problem.