You can view all your transactions and view all your rejections in the "Manage Invoices" section. There you can see all your invoices including the ones that "Require Action",when you view the invoice you will find the rejection/explanation codes in red at the top of the page.
A second way, is to go to Reports and select " Rejection Reasons Report", there you'll see submission rejection code totals,the explanation codes and a full description of the codes. This is all updated daily.
You can also get to your Rejections via the Dashboard. Just click on the pie chart in the Red "Requires action" spot and it will bring straight to that tab. You can click on any color on the pie chart and it will bring you straight to the selected area.