If you forgot your username to the merchant or affiliate panel, you can retrieve it within the login page to both panels:
there, click the link I forgot password
on the following site, please enter your email and captcha image details to the bottom field.
Your username and password with further instructions would arrive to your mailbox after you click Request New Password button.
The email contains link you need to click. It will lead you to the form where you have to fill in your username (email), new password, new password again and proper code from captcha image
Validating this form you will change the details and you will be able to use new password to login to your account.
It could happen that during the installation or inside the merchant's panel you entered a wrong email. In that case, you can not get any password to such account. You can retrieve or/and change the username and password directly in the Post Affiliate Pro 4 database.
Locate the table qu_g_authusers
. There you will find usernames and password of merchant(s) and affiliates.