When you click 'add action' from the Action tab you aren't provided the proposal field. But that field is available when adding action from the proposal tab. I suppose I understand the logic, but it makes more sense for the add action screen to have the same fields available regardless of where you enter it.
Danny, I think the proposal field shows up when you add an action anywhere, as long as there is a proposal already on the constituent's record. But I could be wrong. I just tried it a couple times, and that's what it looked like.