Hi Tjitske,
Welcome to the forums. Another Dutchie, nice!
The problems you describe are very common. TG2 is unlike TG0.9x. Some functions, like the brush you mentioned, aren't present anymore.
There's a painted shader which allows you to paint masks on the terrain, but it does not *directly* alter the terrain like you could do with TG0.9x.
There's a lack of documentation and tutorials which guide you through the whole principle of scene building in TG2, like how you mask aspects of your scene to create desired effects like flattening/smoothing sections of your terrain or layering of surfaces etc.
I have documentation in the form of tutorials cooking, which will cover the very basics and foundations of TG2, so stay tuned.
Cheers,
Martin