The Screen Editor lets you create and update the screens that will form the levels and map of your game.
When we first create a game with MPAGD it will automatically create your first screen (Screen 0). Click on a Block from the bank of available blocks then click in the screen to place your blocks.
Essentially, that's all you need to do to create your screens.
Press X to create the next screen (Screen 1) and add some blocks to it. Make sure that at least the entire bottom row of the screen has Wallblocks (otherwise your player is going to fall off the screen!
You can use the 1 & 2 keys to cycle between your Screens.
Create a few more screens (don't worry to much about them for now, we can come back to them later). In fact, I'd advise you NOT to create all of your screens yet, as tempting as it might be. At this stage it's more important to work on the basics and mechanics of your game first, otherwise you might end up with a lot of pain later redesigning screens that don't really function well with your mechanics.
Next we'll need to add a starting position for our Player on each screen, to do this click the Editor menu and select Sprite Positions.