If you would like to use Arnold in production you can buy a license from Solid Angle. With a valid license key installed, the Arnold watermark will disappear from your rendered images. Solid Angle issues floating and node-locked licenses.
There is an Arnold licensing dialog provided in Maya for the installation of node-locked licenses. It can be found under Arnold > Help> Licensing.
If you have a floating license please refer to the licensing page for instructions on how to install it.
To issue an Arnold license key we need the ethernet address (also known as the "MAC Address", "System Id" or "Host Id") of the computer you will be using as a license server. This is typically a 12 digit hexadecimal number, for example 401c8f464f34. You can use the Arnold Node-locked License dialog to easily copy your MAC address to the clipboard.
Once you have sent us the Mac address and received your license (.lic) file, you can install it here. After installing it correctly, you will be able to start rendering with MtoA.