If you haven't done so already, you will need to download Arnold for Maya (MtoA) from the Solid Angle web site.
After downloading the file, run the pkg (in Finder, double click the pkg file; in the Dock, click Downloads and then click the pkg file).
Agree to the License Agreement by clicking Continue and then Agree.
Select the disk where you want to install MtoA.
Choose if you want to install only the MtoA files, or if you want to configure Maya to use MtoA.
When the installer is ready, you will be asked for the admin password.
You should now see a message informing you that Arnold has been successfully installed.
To load MtoA in Maya, click Window. Settings/Preferences > Plug-in Manager
...and then load mtoa.bundle
There is no automatic uninstaller for MtoA, but you can safely remove the folder where MtoA has been installed if you use Maya 2013 or later. If you are using Maya 2012, you need to edit the maya.env file as well to remove the references to MtoA.