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.
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.
If you're still having issues on your current browser (and pop-up are definitely enabled), please download either Google Chrome or Firefox browser and use Simple Salon through those browsers: Using more than one web browser with Simple Salon