Where do you live?
Do you have a Dessid router? If you do not have a Dessid router (which I believe is only sold in Ireland) then the app is worthless to you.
There are only a few applications that you can install that can decrypt wifi passwords...but they only work with certain routers(and certain encryptions)...and there should be a list of working routers within the app...except Dessid,...because well....no list is required...it's for Dessid routers so a list is unnecessary.