Painting skin weights not working

[Anonymous]

Hey, I'm currently rigging a character and i've done the skeleton and used the bind skin tool so it controls the mesh but now i have to paint weights for it and it's not working? like i selected the mesh and selected the paint skin weights tool but nothing happens, it won't let me do anything? it just shows a red cross under the brush, can someone tell me what i'm doing wrong here? thanks!