It seems something is broken about the default keyboard map in X11 on OS X. I discovered this while trying to use an X-forwarded version of Eclipse from our cluster and couldn’t get any of the shortcuts that use the Alt key to work. Fortunately I found a nice fix in the Inkscape forum.
Enable “Use the system keyboard layout” in your X11 preferences.
Open an xterm. Run xmodmap -pke > ~/.Xmodmap[/code]
Edit the file. Find the lines that say:
keycode 66 = Mode_switch keycode 69 = Mode_switch
and change them to:
keycode 66 = Meta_L keycode 69 = Meta_R
Disable “Use system keyboard layout” and “Emulate three button mouse” in your X11 preferences
Now your alt key should work fine in your X11 apps.