If you've clicked the Buy Credits link in Admin > SMS & Email you may find yourself on the following page, asking you for login details:
If you've been directed to the above page, you'll need to make sure that you're on a desktop (not a phone) when you click Buy Credits.
Also, ensure that you're using a reliable browser like Google Chrome or Firefox.
If you've successfully followed the above guide, and you're still being asked for login details, you'll need to make sure that you've allowed pop-ups for Simple Salon. Click here to learn how to allow pop-ups.