If you load your file from before you applied the displacement modifier, you can adjust the brightness/contrast settings of the displacetex. If you set the brightness to 2 and contrast to 0.1, you can see the plane deforming so the shape you painted (use the middle mouse button to adjust the 3D view). If your plane remains flat, maybe there is something wrong with the settings of your displacement modifier. If your plane does get deformed, try rendering it, it should display some white areas now.
Does not work, i did EVERYTHING exactly step by step but get this render result... pls follow your own instructions and you will see that it does not work http://www.pic-upload.de/view-24770929/RenderResult.png.html
Still works for me. It looks like you forgot to uncheck the checkbox behind the texture in step 9. This causes the displacetex to render instead of the rendertex
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http://www.pic-upload.de/view-24770929/RenderResult.png.html