To install Joomla under your domain, please follow the following steps

1)    Login to Plesk 10.x control panel.

2)    Click on “Domains” and then Click on “Control panel” in front of the domain whose files permission you want to change.

3)    Click on “Applications” icon.

4)    Click on “Install” Button on Joomla to install the application.

5)    Accept the “License agrement” and click on “Next“.

6)    Select the domain under which you want to install the Joomla.

7)    Enter the required details and click on “Install“.

8) You will get the notice “Information: joomla was successfully installed at http://test.com/joomla.” once Joomla is installed under your domain.