That looks like a client validation message. So it is probably on the field itself. If you post a URL I could take a look.
1) The wizard shouldn't be re-run after customization. The individual components on the page can be edited through server behaviors or dreamweaver directly.
2) You can do it however you want, but usually people do Add to cart - > View and update cart - > Checkout
I'm not sure what you are editing or where. It might make sense to give me FTP information. Concentrate on one problem at a time and post specific details here and I'll help you through it.