do you have a MYBT account. this is not the same as your username and password you use for email etc if not you need to set up a MYBT account by going to BT.com
if you don't have a MYBT account then I would use your pc and set up an account at BT.com and then try the app again
after download complete it asks if at home and if you say yes then it asks for your BT.com login which is the sign in for MYBT
are you the bt acount holder? android or iphone?
I use an S2 and have had no problems - even when trailling. when you go to MYBT do you see a link to smarttalk which you can open?