Digital BW, The Print

I've got the book and really like it.  Go back to it often;however on 
page 243/245 there's a procedure of 'saving shadows'.

I've done it successfully in the past but for some reason, each time 
I return to it, I've forgotten something.  After inverting the alpha 
channel & applying the curve for it, I then try to select it so I can 
go back to the layers pallete and do the curves adjustment there BUT 
I get the 'not enough pixels' selected error...usually bang my 
keyboard for a while and happen into it enventually.  Would like to 
actually know it.

Did I make ANY sense?  Not likely.

Sincerely,
Cleavis