Ok, I tested it on my system and although the setting was their, re-reading the question I think you were looking for global zoom.
If you go into System -> Keyboard and choose the "Shortcuts" tab under "Universal" you can see & set the short cuts for Zoom in/out here.
![Keyboard Shortcuts for universal]
However that didn't seem to work for me when setup shortcuts for Zoom in/out. :(
What I had to do to get it to work was.
1) Install Compiz Config Settings manager from Ubuntu Software Center
2) from Compiz Setting go to Advanced Zoom Setting
![Advanced Zoom Setting]
3) Add some keyboard shortcuts for zoom (I use alt+super+= and alt+super+-)
4) Optional scroll button from mouse zoom ( I user alt+super+button4 and alt+super+button5)
![zoom key shortcut setting]
5) exit out of compiz and you should be good to go
OK so the system won't let me post the photos (although it would let me upload them and preview them) I don't have enough karma or some kinda bs as an anonymous question answerer. Strange that it just wouldn't let someone with enough karma approve, oh well. Yah not going to join, so sadly you don't get to see the photos unless someone else puts them up. but you can just read the instructions and that will work.