You will be happy to hear that gProject makes this so simple a child can do it (granted it would have to be one smart ars kid but I stand by my claim ;)

Okay, go into your library and select your Main MovieClip, go back to the gProject panel and select your class.  Then click the dropdown icon at the top and navigate to Utilities>Update UI Elements and click on it.

gProject looks at the Main MovieClip and sees that it has an instance of TestObject in it.  Further it sees that TestObject has a Class and then it imports, into the TestObject Class.  Take a look at and you will see this:

import com.rezn8.ui.TestObject;

 private var testObject:TestObject;

So now you have access to TestObject.  So in the constructor put this:

private function configUI():Void {
  testObject._alpha = 50;

When you run the movie you will see that TestObject now has an alpha of 50%.

Thats it we're done.