Photoshop Design Help Please!!!?

I'm making a website design layout on photoshop. On the tutorial this is what it says to do to get rid of the overlap corners on the header;

''Select the area of your background layer (ctrl + click layer icon in the layer's window), and invert the selection (ctrl + shift + i) and hit delete on your keyboard, this will get rid of the overlaps on the corners.''

I've tried doing that on Photoshop to get rid of the overlap on the header corners but it says ''Could not complete your request because the content of the layer is not directly editable''

How on earth can I get rid the the corner over lap. Here is the image of it and what I want to get rid of!!

LINK:

http://img168.imageshack.us/img168/6704/...

Photoshop Design Help Please!!!?avast

After you invert, select your header layer, then delete

Try that....