To track time for your clients, firstly you have to create some in the Clients section of the account:

1) Click on the Clients section on the side menu;
2) Create your clients by clicking on the "New Client" button. You can also assign different colours to your clients to sort them out.

Once the new client has been created, move to the Project section, on the sidebar menu. Click on the project you would like to associate with your new client and, once you are on the Project overview page, click on the Edit button in the top right corner.

Here you will find a section called Additional info: expand the Client drop-down menu to select the Client you have previously created.

Please note that a project can only be assigned to one client at a time.