GIMP Version: 2.8.14
Operating System: Mac OS
OS Version: Yosemite 10.10.1
GIMP Experience: Beginner Level

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reetings,

I am a new user to Gimp. I am using Gimp 2.8.14 with Yosemite on an iMac. I have been reviewing YouTube videos and reading other tutorial material on using Layer Masks. There is one video which I understand and follow as to the instructive use of Layer Masks but can not duplicate on my current system. I have followed other tutorials and can replicate the process demonstrated in them and it appears that the instructions are similar but the process does not yield the expected results on my system. Consequently, I am a bit confused on how to invoke Layer Masks and use them constructively to manage my images ( mostly digital phots).
The video , if I understand it correctly, instructs one to:

1.Open the image.
2.Make a back up copy in layers.
a. (since the second layer automatically adds a Alph Channel, I did not add one)
3.Select and activate the copy and add the Layer Mask.
4.Make sure the Layer Mask icon is highlighted and activated in the Layer dialog.
5.Select the paintbrush making sure that Black is selected as the Foreground color.
6.Brushing along any area of the image to select it should reveal the transparency layer but does not. I can see the black lines, I'm drawing on the center image on the Layer Mask icon in the Layers dialog, which is white.


I don't know if I creating a problem with the brush selection but the brushes work otherwise. The Alpha Channel does show up in the Channel Dialog box, so I know Alph Channel is present for the image.
I have reviewed some problem solving for Gimp not performing as expected and have made sure that “Show Selection” is activated in the View Menu.


Any thoughts or suggestions will be appreciated,
Henry

The only thing I can think of is that because you are working on a duplicated layer you still have your original layer still on, (click the eye icon next to layer to make it visible/invisible), therefore as you are painting on your layer mask and creating the transparency the original layer is showing through making it appear as though you haven't achieved anything.

2.a is outdated and no longer applies. It can be ignored, however, ...

there needs to be a 2.b added which instructs you to hide the visibility of the original layer (after making the copy).

Thanks so much for the simple solution. Indeed, following your suggestions solved the problem.
Thanks,
Henry

Oh... too late for me to add my 2 cents.
Layer masks are fun.