In most cases you should click on the "Forgotten your username or your password" link under the login form. If you have forgotten your username, entering your email address will generate an email including your username. If you have forgotten your password, entering your username will generate an email with a link to allow you to reset your password.
Any accounts that have been created through a link to an institution's active directory will have to have the password changed by the institution in question. These passwords are not set in the PebblePad software and we are unable to change them. PebblePad Administrators within your university or institution will be able to confirm your username.