flash actionscript: I am linking a movie clip with a class the extends it.
The constructor of this class should get parameters.

I'm not certain, but I believe you can not pass values via the constructor.
The only way I know to achieve what you want is to use the Initialize
functionality of the attachMovie(). Try this instead. (code may have bugs)

var myClip = _root.createEmptyMovieClip("theInstance", 1);
myClip.attachMovie("MyClass", "theInstance", 1,
{myProperty1:"hi",myProperty2:0));

// asuming your class had two properties, myProperty1 and myProperty2, they
will be initialized upon creation.

Does this help?

- Omegus Prime Development Team