Two Problems
- Inkscape runs in X11/XQuartz (=Alt/Cmd are messy)
- German localization overlays Alt+D binding for »Clone«.
Activate second-to-last option in the Input tab of XQuartz preferences. In German it's »Wahltasten entsprechen Alt_L und Alt_R« (so probably: Option Keys map to Alt_L and Alt_R). Alternatively you can put the following four lines into your ~/.Xmodmap
keycode 66 = Alt_L
keycode 69 = Alt_R
clear mod1
add mod1 = Alt_L Alt_R
By the way: I put the following stuff also into that file to map Cmd to Ctrl
keycode 67 = Meta_L
keycode 63 = Control_L
keycode 71 = Control_R
clear mod2
clear control
add mod2 = Meta_L
add control = Control_L Control_R
Now Alt+D works for cloning, in principle at least – in the German localization Alt+D is for »Datei« and is the main function for this shortcut. Open up Inkscape.app/Contents/Resources/keys/default.xml and put
<bind key="c" modifiers="Alt" action="EditClone" display="true" />
somewhere after the Alt+D binding, so probably at line 310.
Now Alt+C clones stuff.