Login in Plesk Control Panel.
Go to Domains , it will list domains in control panel .
Find & Click on Manage Hosting in front of Domains.
Click on Applications , it will list the applications.
Click on All Available Applications > Collaboration
Select the PHP BB application and click on Install.
Check to accept term & condition and click on Next.
Specify the location where PHP BB will install and fill the setting information and now click Install.
PHP BB has been installed. Click on phpBB for admin section & Click on Forum for Forum website.