Probably a simple question for ya...

Question

Ok, so I will start this by admitting I know almost nothing about AS3 and very VERY little about programming as a whole. 
I'll post my code below and you'll probably think I'm an idiot for not knowing what's wrong with it, but that's ok, lol.  I keep getting "1120: Access of undefined property..." with a whole bunch of different problems, all involving the movement of a character that I'm trying to create for a game I'm attempting to make.  If you can please tell me what I'm doing wrong and if possibly explain it as if I know nothing about programming, it would be very much appriciated.  I've been working on this for a while now and am not getting anywhere, and I just know the answer is something very simple I'm missing.  Once again thank you very much in advance for an answer,
package
          import flash.display.MovieClip;
                    public class Shooter extends flash.display.MovieClip
                              public var WalkingSpeed;
                              public function onLoad()
                                        WalkingSpeed = 5;
                              public function onEnterFrame ()
                              if(Key.isDown(68))
                                                  _x = _x + WalkingSpeed;
                              if (Key.isDown(65))
                                                  _x = _x - WalkingSpeed;
                              if(Key.isDown(87))
                                                  _y = _y - WalkingSpeed;
                              if(Key.isDown(83))
                                                  _y = _y + WalkingSpeed;
Also a quick note is it worked great in AS2 with a few differences but I was told to use AS3 and when I changed it over and rewrote a few things it no longer works, thank you once again.

Answer 1

Whenever you get errors, you help others to help you by including the complete error messages in your posting.  I cannot say for sure that the following line is a problem, but you can try changing it to see if an error goes away...
public class Shooter extends flash.display.MovieClip
change to
public class Shooter extends MovieClip
Then you will need to make some changes to update the code to be relevant to AS3.  The Key class does not exist in AS3, so all of your Key.isDown() code won't work.
Also, _x and _y are x and y in AS3
You're best bet will probably be to search Google using terms like "AS3 Keyboard move tutorial" to get the info you need to do what you want in AS3