I suggest you try reading the tuts more throughly... check the box in the texture editor bar too the right, where it has paint mode, with 3 buttons under it being detail, colour and both, with detail clicked on at the mo, click on both then try painting again... again it says this all in the tut and if it didnt, I probably wouldnt know it...
try the folder buildings after clicking on the folder static objects.
Smeg: Fuzz commands a lot and promotes good teamwork, db flips vehicles, Bosco is an all-round nub, Outlawz is an SP whore, Nickbond is a friggin ninja and Rhino gets kicked for being a n00b.