NPM needs a DB, so you are definitely using a DB. If you used express installation, I recommend to re-run configuration wizard for NPM to define username and password to your database and use those in WHD.
On the machine you have NPM installed on, go the following file:
You can open this file with notepad. It will give the username\password in plain text for your SQL database.
You can use these credentials import your assets. If you need the instance name, download SQL Server Management Studio (this is also useful for making backups of your database).
Usually the name is MACHINE\SOLARWINDS_ORION
Hope this helps.