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