You will receive the error message wrong credentials if the mobile number or the email address you entered activating the app do not match with each other (for example, the mobile is associated with a Messagenet account and the email to another account).
The first tip is to check if your phone is actually validated on your Messagenet account. To do this you need to login on Messagenet website and go to page: https://www.messagenet.com/utente/sms/
If the mobile phone is actually validated, try entering just this during MTalk activation, without specifying any email address.
Finally, if you still can't solve the problem, write us an email telling us the mobile phone number and the email you were inserting during app activation.