like i said, maybe the opacity is 0% or maybe the mode is on clear or maybe u click the wrong mouse button B)
Hold on let me see through your eyes and get on your computer... oh wait I can't. Post screenies (Press PrtScrn on keyboard then click new the press CTRL+V)
Dont worry I helped him. Turns out he was trying to brush on the white background layer. He didnt know about creating new layers.