Using the application is as easy as opening it, and without having to click any additional button, wait for it to find all the available networks nearby. The process can take from a few seconds to several minutes, depending on the amount of networks that are available.
Once it is finished and you have selected the network (it's important that you don't do this with an unknown network, as it can be considered a crime), and test its security. Or anything else you want to do.
WifiPass is an extremely easy-to-use utility and can help solve problems with your own connection, the consequences of using it inappropriately can be very negative. As it's often said, use it at your own risk.