Make mask from black & white layer?

I have a stock image which came with an alpha mask as a separate black and white jpg. I want to put the black and white alpha jpg on the main image as a mask, but I can't figure out how to make a regular bitmap layer which is black and white into a mask. I don't seem to be able to use this method: < http://photoshop.weblogsinc.com/2005/08/16/photoshop-tip-usi ng-one-layer-to-create-a-layer-mask-for/> because that seems only to work with transparent areas, not black and white. Any Ideas?

Thanks

There are a number of ways to do this. To brng it in as a mask, select the layer you want to add a mask to, Alt+Click on the mask icon in the layers palette to make the screeen view only the mask. Copy your alpha channel from the other file and go back to the image file and paste it in. Then Alt+Click the mask icon again to restore the view to the composite view.

Another method is to open the Channel Palette, click the contect menu to add a new channel. Then copy and paste your Alpha channel from the other file into it. You can then load this new alpha channel as a selection and turn it into a mask on any image layer.

Regards, MM

Thanks - The alpha channel method worked :)

However, for reference, the alt-click/pasting option did not work - I could not paste a black/white image onto the mask layer. It does the same thing as the method described in the link I posted before - I think that method only works if you have a layer with transparency, which you want as a mask and the transparent bits will affect the mask.