I have a small problem that won't go away. When i paint the ground textures in CS it looks ok in the Render Window, but when i run the game, the grass (the actual animated grass) is growing on some places where it shouldn't. Like in the middle of the roads, or trough the rocks, etc.

Pictures: http://www.shrani.si/?3T/hs/7lkCQa/screenshot74j.jpg, http://www.shrani.si/?j/qb/3zesQZrW/screenshot70j.jpg

Is there a convenient way to erase/control where the animated grass should grow? Help is greatly appreciated.

There is no way to exactly control that. Grass grows around the place where you pasted a grass texture and not only in exactly the spot where you painted it. If you want to make sure that no grass shows up in a certain spot you need to use textures without grass around that spot. A road with a grass textures directly adjacent to the road texture will be partially covered by grass. Paint the road texture, then paint a small line of no grass texture next to the road and then paint grass textures at least 100 units away from the road. A brush setting of 2 (or even 1, you need to try) with high opacity should be enough to paint the 'border region' next to the road.

Make sure the opacity on your road texture is at 100% to completely blot out any grass texture that might remain underneath. Sometimes even trace amounts of grass texture spawn grass. As Phitt said, place no grass grass textures on either side of the road. On an untextured cell, I usually lay down a strip of no grass grass with a 4 brush and 100 opacity, lay down a road in the middle of that with a 2 brush at 100 opacity and paint grass with grass outside of that strip with various textures and opacities.