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.
If you don't have an Autodesk Account, click CREATE ACCOUNT
After you log in, you can start a trial
Click the x icon on the top right corner to close the Welcome to your trial dialog box.
You'll see the status of your trial license. Happy rendering!