OK
Now it works on my Mac (10.5.8) but not on the other one (10.4.5).
But "tant pis", i'll work on the first one.
Thanck you for the time spent to your answer

The plug in folder is in the Mac directory. I tried to create one in the User library, but it didn't succeed anymore.

The real message is (in french)
"Erreur de chargement du fichier sketchyphysics.rb
Failed to load /Library/Application Support/Google SketchUp 7/SketchUp/plugins/SketchyPhysics3/dl.bundle"

It's seems to be the dl.bundle file that is problematic?!

@tfdesign said:

Sandeman, have you at any point installed Podium for Mac in the past?

No, i swear.

Sketchup doesn't recognize the file. I tried changing the code and write load "sketchup" at the begining of the code, nothing happen. I just have the tools present in the workspace.
Mystery...

I really start to believe that this Ruby plugIn doesn't work on a MAC. I test a bunch of solution, read all the tutorials and forum existing on the subject.
My Mac doesn't want to work with SketchyPhysics, and i cry.