endergrls idea isn't bad if your used to using ajax, or the like, to talk to your server, but you can talk to the server directly from AS3 and mxml.
You'll need to use the flash.net package.. here is an example of sending data to the server:
ActionScript Code:
private function submiter(inp1:String, inp2:String, inp3:String, inp4:String):void
{
var holdresponse:URLLoader = new URLLoader();
//the responsecompute function can be set up to do whatever with the server response
holdresponse.addEventListener(Event.COMPLETE, responsecompute);
//this event for if there is an error
holdresponse.addEventListener(IOErrorEvent.IO_ERROR, errorsay);
var therequest:URLRequest = new URLRequest("serverpage.aspx");
var varstosend:URLVariables = new URLVariables();
varstosend.oner = inp1;
varstosend.twoer = inp2;
varstosend.threeer = inp3;
varstosend.fourer = inp4;
therequest.data = varstosend;
therequest.method = URLRequestMethod.POST;
holdresponse.load(therequest);
}
this is just a general example of sending data to a server within AS3
hope it helps a bit
in your mxml it can be included within <mx:Script> tags