One more thing: as for the sound, you may want to check out the GreenfootSound class that is included with the new Greenfoot updates. Those can handle mp3 files.

Ah, an update! I'm glad to see that the first part of the message helped. Now to the second part, To allow for scrolling, or at least to allow for objects to move outside the canvas, go to the world, and where it says super(width, height, cell size), add ,false behind the cell size. So say for example super(800, 600, 1) would be changed to super(800,600,1,false). This will allow the engine to handle things outside the screen boundaries. Next, to actually scroll, you will need some sort of variable to keep track of where you're scrolling at. I suggest looking at other scrolling world games. How I worked it was to store an variable for x and y value of the camera, while storing a realX and realY values to the actors to take note of where they are actually located. When moving the camera, x and y values will change, and make it so that all the actors are oriented according to their realX and realY values relative to the camera.

You also might want to enumerate the specific problems (e.g. walking animations not showing), preferably on the discussion. Maybe that will help you.

Great! I myself had my own health bar but this is really nicely done, providing plenty of flexibility. This deserves a vote, and it got mine.

Hmm, ok. First problem is the infinite air time. I suggest using a boolean value to check if the character has jumped (or int value or multiple values for multiple jumps). The walk animation seems to be nonexistent. You may wish to use the frames and framechange values. I have done some work involving that with some of my games, you might find that helpful (all while I work on the new one)