Very old post but I will add to it anyway as I was having the same issue. My problem was that I had already attached my text to a path earlier and forgot about it. Since the path was fairly normal and I was not working with a lot of text I didn't notice for a while. So attaching text can only happen once per text box as far as I can tell. If you made the same mistake as me, that could explain the greyed out attach to path.