Weight paint mode wont paint on the mesh So i downloaded this fully rigged model from blendswap,

and i made adjustments to the model in edit mode, i using the knife too made some cut to imitate that of a shoe to the model and thought that i would weightpaint on manually if the weights are not applied to the new vertices.

back in pose mode.. i pose the rig and confirm that the new vetrices did not get the corresponding weight of their neighboring verteces automatically, so i enter the weight paint mode to fix it and

the weight paint wont draw anything, none of the brushes work, i cant get to fix it!!
here is how it looks


here is the .blend
To do this you need to select which vertices you need to paint first then paint them. So select the vertices as shown below using shift + right mouse click, then paint them using left mouse click. Updated .blend file can be found here.