MacOS (emacsformacosx) c-spc, does not run set-mark-command

[Uwe Brauer]

Hi

Does anybody know how to achieve the traditional binding of 
C-space to set-mark-command on MacOS (for emacsformacos)?

Shift arrow down works, but will not work for a lot of features in org
mode.

Thanks 

Uwe Brauer