Once your RFP is published, the Add New Section button along the lower left becomes Add Addendum.
Once the Add Addendum link is clicked, the following view will appear.
When you create this new Section, it'll be just like other Sections you have created in the past. You'll give a title, and assign a Default Writer and Default Approver for the the content. You can also choose a prior Section with the From Library option which will import for you.
Once you approve this Addendum, a pop-up dialog box will appear asking you if you'd like to notify your participants. Click yes or no to send them a notification that the Addendum has been published and is ready to go.