You can manage your folders/clients in the account menu on the left.
Expand the menu to add an additional folder/client. Or, click on your account menu option at the top right.
At the very bottom of the page, click '+Add Sub Account' on the right.
Fill in the sub account information:
The Title you chose will be select-able from the left hand menu.
Click on it to switch your dashboard to your 'Sub-Account.'
Once you are switched over to one of your sub accounts, it will have its own image templates, message templates, campaigns, order history, and return address.