Merci à tous, j'ai la réponse "officielle" de SU, Jean avait raison!!!
"Hi Albricolo,
Thanks for your reply. At this time, the Mac version of SketchUp Pro only supports one row of toolbar icons. However, each set can be turned into a floating bar. This is a great feature request and we will take your request into consideration as we continue to improve SketchUp.
Regards,
Tommy
The SketchUp Team"