What are the settings to get both AT&T email and GMail on my Nook Color?
why can'nt i get my att.e-mail account to open
There no "settings"; you have to log into the web interface of your email provider, and from there you can read and send email.
You should use the mobile version of each provider's web site:
As for AT&T, you'll have to call them to find out the mobile web address for getting access to your email online.
Remember, the Nook Color is a tablet-device; it's not a computer, so don't expect it to do everything your desktop or laptop can do. You'll be disappointed if you do.