Hey Pierre,
Clearly you had something odd going on with your installed version of ds.
@CJK has shown you how to use an absolute path, but you should be aware of the other ways you can manage executable paths in Keyboard Maestro.
Be aware that the wiki has at least some information on every command and function and usually has fairly detailed information.
Execute a Shell Script action
See the subsection on paths:
https://wiki.keyboardmaestro.com/action/Execute_a_Shell_Script#Path_in_Shell_Scripts
Create a PATH Environment Variable for Keyboard Maestro and Add /usr/local/bin to the Default Path
-Chris