Photoshop :: Changing Background On Car Photo

I'm on a Mac 10.8.2 w/ CS6 13.0.

This is one of the tutorials that seems very straight forward(except for a couple typo's) but when I try it, I'm told the file could not be copied to clipboard because it's too large...however it does appear to paste the car over the bottom photo? Or did it?

"First open one of the shots in PhotoShop and select the entire image by pressing Ctrl-A, then cut or copy the image into the clipboard (Ctrl-X or Ctrl-C, either will do the trick), now close this image ... without saving ! Very important, we don't want to change the original image file, you never know you might need it again in the future.

Now create a new document (Ctrl-N) and just click 'OK', no need to change any of the settings, they are set to the content of the clipboard anyway so I can leave them 'as is'. Once the blank document is there hit the Ctrl-V to paste the first image into place. Time to open the second exposure, again Ctrl-A to select the entire shot, cut or copy the image (Ctrl-X or Ctrl-C) and close this file again (Ctrl-W)... do NOT safe the changes to keep the original image intact !

With this second image closed again I am automatically returned to my new document that already contains my first exposure, just hit Ctrl-C to paste the second exposure on top of this one and I'm halfway done already. The order of these shots isn't really that important, you will be exposing parts of one shot into the second one, which one lies below doesn't matter anyway."
 
Let's say the screen shot below is workable, if not I'll do it the correct way BUT how do I make the brillant layer masking work from here. Everything I've seen says to select the mask, then select the brush, then type of brush, make sure white is the foreground and black is background and just paint. When I paint nothing happens to my images?
 
Then I've tried going to File>Scripts>Import as Stacks and this seems to bring them in but w/ transparent layers underneath,