BTW, congratulations on your new lens! Could it be that the reason it is manual-focus, is that your D3100 doesn’t have a built-in focus motor?
I got quite a bit of use out of my Sigma 70-300. Mine was called a “macro” lens, and it actually did quite well in that regard. Of course when doing macro, I used manual focus anyways, but auto-focus is nice for every-day shooting.