First go to Extensions on your Plesk panel, click on button "Extensions Catalog" and find JXcore Plesk Extension for Node, click on install button and wait. After this extensions has installed, create a subdomain if you need to use php on website and NodeBB has your forum.
Upload NodeBB to you domain root folder, like httpdocs. Go to Extensions again and click on JXcore Plesk Extension for Node to open the configuration page. Click on Domains tab, and select what the domain you need use NodeBB. Change index.js to app.js and click on Ok button. Go to Restart Manager tab and undo Enable Restart Manager and click on Ok button. Got to NPM Modules tab and on Install New Module field, write nconf and click on Ok button and wait. After installed nconf module, you need to install Redis or MongoDB. I recommend follow this tutorial and install Redis: http://sharadchhetri.com/2014/10/04/install-redis-server-centos-7-rhel-7/
Go back on Configuration tab and click on Enable button and wait. You are done. Open your domain and NodeBB installer show on your page.

Sorry if was confused, I'm not very good with english language.