This post is a culmination of many answers combined together. So I want to make sure I understand.

In order to add an mc to the stage from the library, you refer to it by its name in the library. The way all of this is done is:

var someMc:String = "this_mc";
newClass = getDefinitionByName(someMc);
mcAdded= new newClass ();
addChild(mcAdded);

If I wanted to name the mcAdded before its added to the stage would I then use "mcAdded.name = someName" before the addChild?

Here is where things have always been fuzzy. If I wanted to edit the mcAdded after it's been added to stage, I would:

var someString:String = "mcAdded";
var selectedInstance = getChildByName( someString);

selectedInstance.x = 210;

I've had problems with accessing the instances on the stage in the past. It seems the names of MCs I add to the stage via code are usually messed up. Sometimes their called something like "instance 47". Sometimes [object mcAdded]. How do I fix this so I can guarantee that the names of all future instances are predictable? Is there a better way to handle all of this?

[Isocase]

If your working from the timeline and want to add a mc from the library to the stage just give it a class name and instantiate it.

// Mc in library has a class name of Foo and exported on the first frame

var f:Foo = new Foo();
f.name = "foo is my name";
addChild(f);
trace(f.name);

A couple of times working in AS2 I would receive names like instance47 and some other weird names. I usually just re-start Flash and it seems to work fine.