i tried it with your files, i couldn’t duplicate your exact problem, but the “correct” settings for the mask image appear to be to turn off all the use/calculate/invert alpha checkboxes under image sampling, and either set the alpha influence to -1, or 1 with “negative” checked in the influence panel. then just make sure you have transparency turned on for your material and the alpha for the material set to 0. either that or set the material alpha to 1 and set the alpha texture’s influence blending to multiply.
it can get confusing because as you can see, there are several different ways to do the same thing. its helpful to have these options to build more complicated materials, but if you’re trying to learn, the having half a dozen checkboxes that need to be in agreement with each other can easily become overwhelming! good luck!