Create a movieClip on your stage and name it mcDot.
Delete the mcDot from your stage.
Open the library panel, right-click the mcDot clip then select properties:

addChildMenu

Under the "Linkage" category check "Export for ActionScript"
Give the class the name "Dot". Make sure it is capitalized.

addChildProperties

Actionscript:

var dot:Dot= new Dot;
addChild(dot);

dot.x=stage.stageWidth/2;
dot.y=stage.stageHeight/2;