Sorry, forgot you dont have a file to edit! (That's why I never appreciated the issue of SaveAs).
Here's a smbios.plist (attached), save you typing it in. Serial # is not an MBA one, was whatever was in the file when I got it. You can edit that if you want it to look right, there are article out there about what's the right format for MBA vs MBP etc.
So, how to get this into your /Extra folder
Many options, I'll just cover one:
1. Terminal. yes the Utilities terminal. Open that. It will "place you" at your home folder.
Will list the files and folders in your current location.
Will list them in a vertical list with more information (owner, permissions, size etc).
Will move your "location" to the root (top) of the filesystem
Will move your "location" to the Extra folder.
Is a shortcut to get you back to your home folder (~), rather than having to type stuff like cd /Users/YourName.
Is Present Working Directory, ie: where your current "location" in the filesystem is. Useful if you forget or get confused
OK enuf of that, should be sufficient for now. If you get into using Terminal there's loads of primers out there.
2. Assuming you download/save/extract the smbios.plist to your desktop..
Assuming also that your present working directory is your home (else do a cd ~).
sudo cp Desktop/smbios.pist /Extra/smbios.plist
You will be asked to enter admin password.
This will copy the file from your Desktop to the Extra folder, as sudo means do this command with super user rights.
3. I seem to recall that ownership and permissions on smbios.plist in /Extra is not a big deal, so should work anyway. But you may want to check and change them...
Look at the columns to the left of the file/directory names.
For permissions, that's the stuff like rwx r-x r--
sudo chmod 755 smbios.plist
Will set the permissions to an appropriate value (not all files/directories do or should have the same of course).
Ownership will be (should be) stuff like root-wheel for most of the files in /Extra. Unlike files in your home folder which may be like me-staff or whatever. Check what's next to smbios.plist. If it's not root-wheel and you want to change it:
sudo chown root:wheel smbios.plist
If you were changing a directory and it's contents (eg: a .kext) you'd use chmod -R root:wheel directoryname or chown -R 755 etc..
Hope that's readable and makes sense, and helps explain where/how you could use the sudo chflags nohidden thing also.