There are couple of things to do before you can paint mesh with texture. I always forget it so here is really quick stop-by-stop process how to make texture painting available in Blender Cycles:
1. Add mesh
2. Unwrap it
3. Add new, blank image texture
4. Add new material with the image texture you've created in the previous step.
5. Go to the texture paint mode
6. Go to the texture tab on the right, select Brush, click new and set up your texture you would like to paint.
7. Paint your mesh with texture