What environment variables must be set on the server?

APPX_SERVER - Full path to the appx.exe on the server.  Make sure
              that you reference a drive that is available from
              the server, not a network drive that is only visible
              from clients


APPX_KEYMAP - Normally set to "Windows", but you can set it to
              a different name if you wish to customize the
              keymap


APPXPATH    - Full path to your APPX data directory

These variables MUST be set on the server. New values will not be in effect for any APPX sessions until the server machine is rebooted.