I don't understand what you are asking. If you had enabled "Find my iPhone" in settings on your phone then you can track it by logging in to iCloud.com with your Apple ID and clicking on "Find my iPhone". There is no app involved.
What are you talking about? You don't have to "activate the app". You activate the service. Once you turn it on, don't turn it off.
The app is used to locate a different iOS device. Or, you can log into iCloud.com and locate it there... provided the phone is switched on, has not been wiped, and is connected to either the cellular data network or WiFi.