Now, I have the "level.cls" file. BUT, it still dont work!!! I added the code lines which are in the NGD Userguide, to make this file. And a save allways before trying the level. And the box still simly falls through the plattform. I tried to make the plattform higher, to put the box directly over the platform, but it didnt help. When the box falls through the platform, it makes a collosion sound, so Newton recognize that there is something under the box, but the box dont stop falling. Sometimes (I dont know exaxtly when, I just made some experiments) the box falls INTO the platform, and stays there and moves in the platform.
What is wrong?

I have the GS6 Extra (Sybex-Version). Is this OK?
Could somebody perhaps make a simpe level, like on the screenshot? Would be nice.