If you have a single-user subscription for Arnold, you just sign in with your Autodesk ID.
How do you know if you have a single-user subscription?
If you use Maya or 3ds Max, you can Sign In in those applications, but you'll still need to use the Arnold License Manager to register Arnold for single-user licensing.
And, depending on whether you also use network/floating/multi-user licensing, you may need to switch to single-user licensing.
Open the Arnold License Manager (in the Arnold menu for your Arnold plugin, click Licensing > Arnold License Manager)
If you don't see a Single-user tab, click License > Switch to Single-user Licensing
If prompted, install single-user licensing and register Arnold.
Click Sign In and enter your Autodesk ID and password.
You'll see the status of your single-user license. Happy rendering!