The Course page does not have a drop-down field for your resources. It's great to list the resources you intend to use in the "Description" field, but you can only manually type it in the text box.
The Lesson Plan page does have a drop-down field for each assignment which should show the list of resources you have already entered in the Resource page. There should also be a "+" button which will allow you to add something not already in the list.
Which page are you referring to?