Why could VirtualBox not find a registered machine named Windows_7

I'm trying to change TCP/UDP of a virtual machine using VBoxManage setextradata.

Whenever I type the command:

sudo VBoxManage setextradata Windows_7 "VBoxInternal/Devices/pcnet/0/LUN#0/Config/guestEmule_TCP/Protocol" TCP

I get the following error:

VirtualBox Command Line Management Interface Version 2.1.4
(C) 2005-2009 Sun Microsystems, Inc.
All rights reserved.

[!] FAILED calling a->virtualBox->FindMachine(Bstr(a->argv[0]), machine.asOutParam()) at line 3688!
[!] Primary RC  = VBOX_E_OBJECT_NOT_FOUND (0x80BB0001) - Object corresponding to the supplied arguments does not exist
[!] Full error info present: true , basic error info present: true 
[!] Result Code = VBOX_E_OBJECT_NOT_FOUND (0x80BB0001) - Object corresponding to the supplied arguments does not exist
[!] Text        = Could not find a registered machine named 'Windows_7'
[!] Component   = VirtualBox, Interface: IVirtualBox, {339abca2-f47a-4302-87f5-7bc324e6bbde}
[!] Callee      = IVirtualBox, {339abca2-f47a-4302-87f5-7bc324e6bbde}

The virtual-machine has been created using GUI. Any idea?

$ cd /Users/marco/Library/VirtualBox/Machines/Windows_7
$ ls
Logs                Windows_7.xml
Windows 7.xml.1.5-macosx.bak
$ sudo vboxmanage registervm Windows_7.xml
VirtualBox Command Line Management Interface Version 2.1.4
(C) 2005-2009 Sun Microsystems, Inc.
All rights reserved.

[!] FAILED calling a->virtualBox->OpenMachine(Bstr(a->argv[0]), machine.asOutParam()) at line 762!
[!] Primary RC  = NS_ERROR_FAILURE (0x80004005) - Operation failed
[!] Full error info present: true , basic error info present: true 
[!] Result Code = NS_ERROR_FAILURE (0x80004005) - Operation failed
[!] Text        = Could not lock the settings file '/var/root/Library/VirtualBox/Windows_7.xml' (VERR_FILE_NOT_FOUND)
[!] Component   = Machine, Interface: IMachine, {ea6fb7ea-1993-4642-b113-f29eb39e0df0}
[!] Callee      = IVirtualBox, {339abca2-f47a-4302-87f5-7bc324e6bbde}