How to install a Plesk extension?
In Plesk Onyx 17.5 all available extensions can be found in Extensions section on the right side of Plesk for all views:
Search for the required extension using the search bar or select any of popular one and click Install button.
Additionally, the extension archive can be obtained from http://ext.plesk.com. Then, uploaded to Plesk in Extensions > My extensions > Upload Extension
After that, the extension will be ready to use and can be accessed in Extensions > My Extensions section.
An installation method via command-line is also available. Plesk autointaller utility can be used for it:
# plesk installer
For older Plesk versions, Extensions section is available at the same place, but all the actions can be done right after clicking it.