During the install of a production version of NPM (a non-evaluation version), you should see a popup asking for your customer ID and password, and other details to download your lciense key.
On an isolated network, you can't downlaod the key automatically from our key server, so you will need to click 'Skip This". You'll be presented with the Hostname of the server,and a serial number for the product. Note these down.
On a server with Internet connectivity log into your customer portal, click "Register your software", and click the 'Manually register license" link. You'll need to enter the hostname, serial number, and some additional details here - your name, company etc, then you can generate the Key.
Copy the key into the space provided in the licensing screen and you should be good to go. If you're still having trouble, submit a ticket to us using our online ticketing form here: www.solarwinds.com/support