Login at the Customer Control Panel with your main account's username and password.
Click on the “Login via IP address” link on the Home tab under the Plesk Section.
In Plesk, click on the “Domains” link on the left hand side.
Select the domain you want to manage.
On the right hand side, click on the “Email Addresses” link.
Click on the “Create Email Address” button.
Type in the email address you want to create.
Type in the password, confirm the password.
Set the size you want to give the account.