I'm having the same problems, got the redeem codes, but no link to that promised extra game, although pre-ordered.
I'm new to GOG, is it possible that we'll get an extra e-mail in the next few hours maybe?

It is always possible if you contact them first. Because before this, there are certain issue in GoG that prevent the email reach some of the customers during the first few days of the pre-order. But later, they resend the email to all the customers.
Just drop them an email.

My link is showing up now on the page it's supposed to be on. Now I have another problem. Which game should I pick? I'm not really an RPG person.

I got a response from GOG support saying they had a problem and it was now resolved so to try again. I checked and the button to get the bonus game was there now.