Help Needed With a Sigma on Nikon

[Mardave Mad]

I've just got a Sigma 70-300. I knew when I got it, that it was a manual focus lens when I got it. I've put it on my camera D3100 and it's telling me to "Lock lens aperture ring at minimum aperture (largest f/-number)

So I've done that, and it still says it. So I had a look, and it appears that there is a black "lug" that needs to be pressed down in order for the lens to "work"

What can I do?!

 

[Mardave Mad]

Think I may have solved the issue. There appears to be a bit missing off the lens. Looks like there used to be a bit that would push the "lug" down and it's no longer there... Anyone got any idea's how much to fix?