In-App Purchases cannot be added to this version because it has already been submitted for review. To manage your In-App Purchases, go back to the App Summary page and click on the Manage In-App Purchases button.

This message in iTunes Connnect:

In-App Purchases cannot be added to this version because it has already been submitted for review. To manage your In-App Purchases, go back to the App Summary page and click on the Manage In-App Purchases button.
is a pain.
The answer is that you can only select an IAP item in the “View Details” page of your app’s new version is when the App Status is in the “Ready to Upload Binary” or “Binary Rejected” state.