To install Atmail under your doamin,please follow the following steps :-
1) Login to Plesk 10.x control panel.
2) Click on "Subscriptions" and then Click on "Manage Hosting" in front of the domain.
3) Click on "Applications" Tab and Click on All Available Applications.
4) Click Show Search.
5) Type Atmail in Seach Box and click on Search and after few second you will see
Application name in the list.
6) Click On Install button.
7) Click on I accept the terms of License Agreement and click on Next >
8) Replace from atmailopen to Webmail if required and click on Install.
After few second you will Notificaion , Your application has been installed.