The Below is from a generic install from the Application installer. Had I put this under httpdocs\subdomains there would be no issue. Notice the path is out side the httpdocs folder
Warning: require(./wp-blog-header.php) [function.require]: failed to open stream: No such file or directory in...
First log in to Plesk then check which domain the application is installed under.
Go to "Websites & Domains" and hover over the domain.
You should see something like https://localhost:8443/smb/web/settings/id/555
We want the Last number: 555
now you will need to login to the psa database...