As an agency you need to create at least one client to be able to work with Mediatool. Do so by clicking the Clients link in the organization menu to the left.
Click create client in the submenu and enter the details of the client in the form.
Once a client is created you will be able to invite users to work with that organization by selecting the client in the Clients submenu.
To start working with your client click the 3 dots next to your organization name in the main menu. Select the client in the organization list by clicking it.
Now you are ready to start media planning for this client.