How can I put a downloaded save game on my Xbox 360?

This should be a simple thing to do, really, but I can't figure it out using a Mac. I completed GTA IV years ago up to 99.75% and now I would like to play it again - but I don't have my save game no more. I really, REALLY don't feel like doing all those missions a 2rd time just in order to get access to helicopters and the other islands.

Using a Mac, how can I put the savegame on an Xbox-formatted USB stick? It's an unmodded 360 slim. I'd be grateful for any help!

 

USBXTAFGUI_v44 or Xplorer360

 

Is there a way to do it with OSX? :/ I do not have a virtual machine.

 

Not that I know of, there's also Horizon but as far as I know everything is on Windows.