gimp layer Masks

I have been looking all over and seem to get confused or cant find this out…
When I have layers in gimp, and I want a predefined mask, how do you load this mask? When I go to add new layer mask I only get an option of 3 defaults. A white page does not help me at all in makeing a mask, I would like to load a black/white image I already made for the mask.
I hope this makes sence.

Open the mask image, select all, copy. Go to your other image, add a mask (just pick on of the defaults). Select the mask in the layer window and paste.

Have fun,

Wybren