There are a few steps to complete in DFP to authorize ADX inventory before setting up the order:
DFP Inventory > Ad Exhange Inventory
1. Click on Inventory > URLS
New URLS > copy the URL from the Next Gen Excel
Setup ADX Order
Use the Next Gen Excel tab "DFP ADX Order"
1. Order Name is:
2. Line Item
Name: use the Excel Col A
3. Line Settings
Just change the Type to "Ad Exchange"
4. Add Targeting
Click on "Ad Units"
5. For the Ad Unit - only select the "APN" if you see both an _APN and _DFT.
6. Click "Save and Upload Creative"
7. Past the Creative Name from the Excel
8. For a New Order you will be asked to "Approve"
9. Finish all the line items on the sheet.