> I have read the chapter "Extending the Squeak Virtual 
> Machine" in the blue book, but there is still some dark points.
> How could I get an oop from a class I made ?

Pass it as an argument into a primitive.

> What would be 
> the Slang/C translation for 'Smalltalk at: #MyGreatClass' ?

Doesn't exist. The VM makes no assumptions about the way the image is
organized.

Cheers,
  - Andreas