A few notes:

• You need root permissions to do the automatic installation.
• Use sudo, not su. You want to install MtoA under your user account, not the root user account. sudo will let you install MtoA with root permissions, but under your own user account.
• If you install as root, the MtoA installer copies mtoa.mod to /root/maya/2014-x64/modules. Unless you’re running Maya as root, that’s no good. You want the mtoa.mod file in your home directory (for example, /home/stephen/maya/2014-x64/modules). That way, when you run Maya it will load mtoa.mod and find the MtoA plugin.
• The automatic installation also copies arnoldRenderer.xml to /usr/autodesk/maya2014-x64/bin/rendererDesc. That allows you to batch render with Arnold from Maya, without having to set MAYA_RENDER_DESC_PATH.