How do you purchase apps on the iPad? I have been successful in downloading free ones but I would now like to purchase one. I. Push buy app and it asks me to sign in. I put in my ID and password, it thinks for a moment and then displays the same dialogue box asking for my ID again. Once I do log in, how do I purchase? I have £10 but think that's for my wi.fi use. Can I use this to but apps?