After installing Serv-U please follow its standard "Setup Wizard". On the first step click "Next" to proceed with the wizard. 

Select "Enable small images" option and click "Next" button.

Click "Next" to start FTP server and connect to it.

Select IP address to bind FTP Service to and click "Next".

When Serv-U offer you to setup the first default domain click "Cancel".

Open Serv-U console, expand << Local Server >> node and select "New Domain" from the context menu of "Domains" node.

Select IP address this domain will be listening to. You may select "Use any available IP address" to listen all server IP addresses.

Specify the name of new domain.

Select TCP/IP port to bind domain to. Default port is 21.

Select "Store in computer registry" in domain type field. Please note this setting is critical for normal DNP communication with Serv-U.

After completing "Add new domain" wizard click "Put Domain Online" button to activate it.

When adding Serv-U FTP service in DNP select created domain name from the steps above in "FTP Accounts Site" field.