help with magnification in 12.04

[Milton]

Hello List,
I use ubuntu 12.04 with Orca 3.4.2 in unity 2D and also installed the 
fallback session. So I also can work with GNOME without effects. I only 
work with speech.
For a couple of friends who wants to try Ubuntu with magnification I 
need your help.
In the keyboard preferences on the tab short cut keys I set:
1. Alt_Control_M to toggle magnification on or off;
2. Alt_arrow-up for zoom in;
3. Alt_arrow-down for zoom out.
But there is no result by pressing those shortcut keys.
What should I do to have magnification work?
Many thanks.
Milton