Comment 4 for bug 1174706

Ubuntu 14.04 / Unity: If I try to set Alt+Print, it just goes back to "Disabled" for me.

Workaround:

First, unset Unity's default Alt+Print action:
gsettings set org.gnome.settings-daemon.plugins.media-keys window-screenshot ''

Launch dconf-editor and go to "org.gnome.settings-daemon.plugins.media-keys".
It you want to change one of the default actions, set your action's value to "<Alt>Print".
If you want to change a "Custom Shortcut" you created via Unity's control center, navigate to "org.gnome.settings-daemon.plugins.media-keys.custom-keybindings", and choose the appropriate "custom*" (where * is a number) folder. Set the "binding" key's value to "<Alt>Print".