You can start any client on one of your workout plans and schedule his or her workouts using the "Manage" link on the "Clients tab" of the Trainer Dashboard:

Click the "Workouts" tab on the left hand side then click the "Start Plan" link: 

Choose the workout plan that you want your client to start, then follow the prompts to set up the workout schedule, enter 1RM info (if needed), choose their start date, and choose the first workout.

Once you're done, click "Start Plan", and your client will be ready to start logging their scheduled workouts!