Adding selection of separate layer to layer mask?

Hi everyone,

I have a stock photo and a separate image file that is the black and white layer mask for that photo, but I cannot figure out how to add the selection of this black and white mask and to the actual layer mask for the photo.


It's been incredibly frustrating trying to figure out how to do this as I am new Photoshop and it seems like such a simple problem to solve. I'm very thankful for any help you can give.

-Drew

Place the file in rgb mode the B&W one, is in index mode