Ok, The problems I found were just a few little mistakes but it is all working now. Here are the issues, and the steps taken to fix them:
1. The reason all you could see was white and the dummy puck is because you had the "Get Ready" Layer 3 set to transparent "No" in the properties window. This is a easy mistake to make. This probably happened when you thought you had the background layer selected, it happens. So layer 3 was blocking out all the layers beneath it. When working with C2 it is a good practice to do your background layer first, and lock it you can always unlock it if you need to make changes.
I set it to yes.
2. You had instance variables both Player_Score, and AI_Score set to your Paddle_AI. This wouldn't have worked, and was not needed since you already had global variables set properly.
I deleted both instance variables.
3. You had the Paddle_AI Set Y to Mouse.Y when it should have been your Player_Paddle. Easy mistake its like making a typo. This wouldn't have worked since the Paddle_AI is your AI (Artificial Intelligence), and is using lerp to move.
I changed it to Player_Paddle Set Y to Mouse.Y.
4. You had:
+Puck->On collision with Player_EndZone
-System->Wait 5.0 seconds
When it should have been:
+Puck->On collision with Player_EndZone
-System->Wait 0.5 seconds
Another easy typo type mistake.
5. After I got it all running good I noticed that your dummy ball, ball, and Play again button all had white backgrounds which just didn't look good.
So I exported your sprites, and got rid of the whit backgrounds with Photoshop. Then I replaced those sprites with the cleaned up ones, cropped, added image points to your ball, and re-sized the ball's to look good. I hope you don't mind.
Anyway here is your fixed .capx:
https://dl.dropbox.com/u/43763487/Pongboy/pong.capx