The first step in making sure everything is setup correctly is to go to Appearance > Themes (or Plugins) and seeing the notification update:
If you only get "New version available" without the "Update now", it means the API details are not correct.
Getting the username:
The username can be found on this page:
Getting the API Key:
The API Key(s) can be seen / generated on this page: (make sure the button on the API key you're trying to use says "Deactivate" (that means the API key is enabled).
Simply clicking on the API Key will copy it to your clipboard and you can move to the next step and add it in your product (Divi / Extra themes or our plugins).
Adding the API details in the Divi theme:
If using our plugins, here are the locations for setting the API details for each one of them: