Once you login to your master account you will see the list of your sub accounts and button "Manage" beside them. By clicking on the button "Manage" you are logging into that sub account and you can trade links and manage all of the sub account's features.

If you want to change the sub account just click on the tab named "Master Account" which is located on top of the webpage and selected a different sub account by clicking "Manage" button beside it.