In Xfce the "Alt" key usage is defined in a completely different GUI than the other keys! One that's not labeled "keys" ... it's so cool!
So perhaps check under "window manager", "window manager tweaks" or something like that, which is where it's hidden away in Xfce.
Mint 17.1/fluxbox/pulse-less - Debian 8/fluxbox/pulse-less - Ubuntu 15.04/fluxbox/pulse-less
Please edit your original post title to include [SOLVED] if/when it is solved!