dJ, in the high pass filter layer, you can create a mask from the one of the icons at the bottom of the layers panel (by default it sits on bottom right of the page) - there's a line of little icons there and from memory the layer mask one is the second from the left. In the last image in the tute, it's got an unlabelled arrow going to it.
part 2 is around but all of the stuff is in the fridge tutorial anyway
thanks for reading!