THANK YOU : )

;==================================================
var worldAABB: b2AABB = new b2AABB ();
worldAABB.lowerBound.Set (0, 0);
worldAABB.upperBound.Set (stage.stageWidth, stage.stageHeight);
_b2World = new b2World (worldAABB, new b2Vec2 (0.0, 10), true);

// debug draw start
var m_sprite:Sprite;
m_sprite = new Sprite();
addChild(m_sprite);
var dbgDraw:b2DebugDraw = new b2DebugDraw();
var dbgSprite:Sprite = new Sprite();
m_sprite.addChild(dbgSprite);
dbgDraw.m_sprite=m_sprite;
dbgDraw.m_drawScale=1;
dbgDraw.m_alpha=1;
dbgDraw.m_fillAlpha=0.5;
dbgDraw.m_lineThickness=1;
dbgDraw.m_drawFlags=0xFFFFFFFF; _b2World.SetDebugDraw(dbgDraw);

//flash stage
var mcCircle: Sprite = new McCircle();//McCircle is a movie clip in the library identified as McCircle
mcCircle.width = 100;
mcCircle.height = 100;
mcCircle.x = 100;
mcCircle.y = 100;
addChild (mcCircle);

//box2d world
var rect: Rectangle = mcCircle.getBounds (mcCircle);
var circleDef: b2CircleDef = new b2CircleDef ();
circleDef.radius = rect.width / 2;
circleDef.density = 1;
circleDef.friction = 0.5;
circleDef.restitution = 1.1;

var bodyDef: b2BodyDef = new b2BodyDef ();
bodyDef.position.Set (mcCircle.x, mcCircle.y);
bodyDef.angle = mc.rotation * (Math.PI / 180);
bodyDef.userData = mcCircle;

var body: b2Body = _b2World.CreateBody (bodyDef);
body.CreateShape (circleDef);
body.SetMassFromShapes ();
;==================================================

but…calculations from box2d tend to be really slow when its units are set to 1:1 pixel ratio.
what i do is simply scale all units when inputing into box2d world…then i scale them all up when translating back to flash stage.

e.g.:
;==================================================
var _scale: Number = 0.1
var worldAABB: b2AABB = new b2AABB ();
worldAABB.lowerBound.Set (0, 0);
worldAABB.upperBound.Set (stage.stageWidth * _scale, stage.stageHeight * _scale);
_b2World = new b2World (worldAABB, new b2Vec2 (0.0, 10), true);

// debug draw start
var m_sprite:Sprite;
m_sprite = new Sprite();
addChild(m_sprite);
var dbgDraw:b2DebugDraw = new b2DebugDraw();
var dbgSprite:Sprite = new Sprite();
m_sprite.addChild(dbgSprite);
dbgDraw.m_sprite=m_sprite;
dbgDraw.m_drawScale=1/_scale;
dbgDraw.m_alpha=1;
dbgDraw.m_fillAlpha=0.5;
dbgDraw.m_lineThickness=1;
dbgDraw.m_drawFlags=0xFFFFFFFF; _b2World.SetDebugDraw(dbgDraw);

;and…

;…
circleDef.radius = rect.width / 2 * _scale;
;…
bodyDef.position.Set (mcCircle.x * _scale, mcCircle.y * _scale);
;…
;==================================================