Hi Steven, you can link to your shopping cart from your website by putting links to the shopping cart home page or any specific page in the shopping cart. The main SC page is "index.php". So for example, you would have a URL in your link of "http://mysite.com/shop/index.php". That's assuming that you put your cart in it's own folder ("shop") on your host.
If you want to link to specific categories, you can get the URL by either hovering over the categories link in the shopping cart or copying the category link in your browser's address bar when you are in that category. For example, "http://mysite.com/shop/viewcategory.php?groupid=1". Or, if you want a specific product you could use, "http://mysite.com/shop/viewitem.php?groupid=1&productid=1". The numbers are SC's way of keeping track of your categories and your items.
Lester, I believe Scott is referring to the information that you give to Paypal, Google, or Authorize.net when you set up your api with them.