If  you already have the Autodesk Network License Manager (NLM) set up for your Maya or 3ds Max licenses, you can use that license server for Arnold too.

Windows

Save your Arnold license file in the folder where you installed NLM.

In the NLM, go to the Start / Stop / Reread tab, and click ReRead License File.

On the Server Status tab, click Perform Status Enquiry to check that the Arnold license is available.

OSX