NSX can only be managed using the vSphere web client. When NSX is configured with vCenter, it installs a plugin for the web client. There is no management functionality available in the classic Windows client.
If you are already logged in, you will need to log out and log back in to the vSphere web client after about 5 minutes for the plugin to show up.
Once logged in to your vSphere web client, you will see a new icon called Networking & Security. You will see this on the left-hand side navigation pane as well:
Now, perform the following steps to generate a license: