OK, since I'm trying to learn to edit because I want to help out, I was working on this tutorial. The part about the luminance mask drove me batty, probably because I'm not very familiar with masks and stuff in Photoshop anyway, and also I think Adobe may have changed things around a bit in CS4 vs. the version petopeto wrote the tutorial based on. I finally sorted it out, and thought I'd share for others who may be confused too, this is step 4:

Create a Layer Mask on the Dark layer you made in Step 3. Make sure you have the Dark layer selected, then either click on the Add Mask button in the Layers box, or do Layer > Layer Mask > Reveal All.



Now make sure you have the mask selected and click on Image > Apply Image. Select invert in the dialogue box, but leave everything else as defaults. If you did it right the mask should change to an inverted black and white image.



Again making sure you have the mask selected, adjust levels (Control-L or Image > Adjustments > Levels) If you do it right the mask will get a bit darker, with less white lines showing. It's really subtle so not worth a screenshot.

Finally, apply the mask to the Dark level by making sure you have the Dark layer selected, then using Layer > Layer Mask > Apply. If everything went properly the Dark layer will become mostly transparent, with only the darkest parts still in it ready to be merged.



Now you can merge the layers, but Photoshop will most likely name the new layer Dark because Dark was on top. Don't let this confuse you in the next step.

Hopefully this will be useful to someone. ヾ(＾∇＾)