Change File Permissions in Plesk
From Lunarpages Web Hosting Wiki
How do I change file permissions?
Customers have total control of their site for permissions (ie, installing 3rd party scripts)if you enable the proper permissions.
You can modify permissions by logging into Plesk, clicking on your domain name, and choosing File Manager.
At the right hand side of the screen, an icon resembling a lock is the permission icon. You must set the appropriate permissions such as Read/Write to whatever the script requires.
We do not provide support for 3rd party scripts other than scripts installed with Plesk.