There is no purchase required for the app, only for the data. However, if you already have a data subscription for your 740, then you can also use a phone or tablet with it for no additional cost (actually two phone and/or tablet devices, if they are both on the same OS).
I think if you launch the app, it might prompt you at startup to buy a subscription or log into an existing account, so you can either follow that path to log in, or else once the app has started you can tap the Menu button, and I think the option at the top of the list is to buy a subscription or log in. (Sorry for being vague...the login option only appears if you're not currently logged in, and I don't want to log out just to see what it says because I have a very convoluted password and don't want to retype it!)
The user credentials are the same ones you would use to log into the iflyefb.com website, not these forums.