From Lunarpages Web Hosting Wiki
The Application Vault in Plesk gives you quick and easy access to install a number of popular scripts in just a few seconds. So if you want to get a new section of your web site up and running in a hurry, and do not wish to mess with manual installs this section of the Plesk control panel will come in very handy to you.
How to Use Application Vault in Plesk
The first thing you must do is login to your Plesk control panel. Once you have made it to the front page, click on the domain name you wish to add the application to, and then click on Application Vault. It is located under the "Hosting" category. On the Installed Applications page click on the icon that reads, "Add New Application".
Here you will see a list of applications you have easy one click access to installing. Click on the name of the script you wish to install. Before you do so, make sure you have selected the domain (or subdomain) where you wish to install it. Here you will see the Site Application Information. This page will tell you a little more information about the application you are about to install.
Once you are ready to install, hit the "Install" button.
Now, depending on the script you selected to setup you may need to fill in extra information that will be unique to that script. You will need to give it such things as a unique user name and password, your e-mail address and more.
If you ever need to install another application, feel free to repeat these steps. As you can see, Application Vault allows you to easily auto-install selected third party scripts into your web site.