Plesk 10 does not support domain administrators, however here is a work around to give similar domain level access to your clients.

Kindly follow the following steps :-

1) Login to your Plesk control panel.

2) Click on "New Customer" 

3) Click on "Add New Customer".

4) Enter the details as requested on the "Form" and click on "OK".

5) Click on "Subscriptions".

6) Select the Subscriptions whome you want to give Domain administrator access.

7) Click on "Change Suscriber" and select the customer you have created above.

8) Once you have selected click on "Next" and click "Finish".

9) Now you can access the control panel from the URL https://domainname.com:8443.