Im not the expert but i thought that the aerobic threshold is where, when plotting the lactate levels on a graph the curve will take a steep increase or acceleration OR when you hit 2 mmol. I dont fully understand the last part because some people may have a high resting lactate levels. I think the books show this quite will with various graphs.
So where the lactate graph takes a change in slope (ie accelerates upward) at the corresponding heart rate will be the aerobic threshold.
Hope this helps.