I want to fill the dent on the deck as shown in the picture (http://
imagebin.org/27673) before painting over it. I'm not sure what the
material is called in English. What is the material that is used to
fill the dent called? Is it plaster (a guess)?
The generic name is floor-leveler and it comes in several different brands. The one I'm personally familliar with is made by KenTile. It may not be available in your area but it gives you a place to start.
Best wishes with your project.