If it's an object on the stage you need to give it an instance name using the properties panel, and then you can access it through the _root property:

_root.myMc.doSomething();
Use of Identifier:
Telling a library symbol to export for actionscript works in a similar way to in AS3. The export name you assign is a name that you use to create an instance of that symbol, using the attachMovie function:
var myMC:MovieClip = this.attachMovie("instanceName","MyMC",this.getNextHighestDepth());
attachMovie accepts 3 parameters:
- instanceName: this is like the
name property of a DisplayObject in AS3. It is a unique name that you assign to each instance of the symbol you create.
- Library Identifier: This is the name that you put into the Identifier field when creating the symbol.
- Depth: The depth of this MovieClip on the display tree. You can target a specific depth, or use
getNextHighestDepth to place it in the highest available depth, same as using addChild in AS3.