Scroll to the bottom of the page and there you will see all of your Merchant Accounts.
Within the Checkout Settings page of the plugin, navigate to the Merchant Accounts section.
Click the button for the environment you wish to add a merchant account.
An empty row will be inserted into the merchant's table. You can then enter your merchant account and the currency associated with the account.
Close the modal and save your settings. Validations occur when you save your settings to ensure you have associated your merchant account(s) with the correct currency.