In the default workflow model, jobs are linked to Projects, not to Clients.

The "New" button for jobs is found in the "Client jobs" tab of the general "Projects" list. 
Please do the following:
1) Click the Projects tab at the top of the screen and the "Project" button.
2) In the window that opens, select the required project, and click the "Client Jobs" tab.

Alternatively, you can switch to the "Without Projects" workflow mode, in which case the "New" button for client Jobs will be found in the "Client Jobs" tab of the client.
To do so, please do the following:
1) Click the "Settings" button near the Upper-left corner for the main window (the button depict the TO3000 logo)
2) Go to "Advanced Settings >> Workflow mode"
3) Select "Without Projects mode" and click "Set now"