You can only choose associations (1..1, 1..n or n..m) that exist in your domain model and that match with your json structure.
So your entity Summary (Root) should have an association in your domain model.
To answer this question:
“I think section 4.8 is the step I didn't understand, it says this: Double-click Import_mapping in the App Explorer and from the Connector pane, drag Input as the input parameter for the input mapping. Where in the Import Mapping screen do I drop the Input mapping so the association is correct?”
You can drop it anywhere on the Import mapping screen. After that you need to click, Map automatically
As far as step 4.10 goes, you must have missed creating an association and in your case, if no association exists, you don’t have to chose that option.
Did you do step 4.7:
Drag an association from Input to Summary.
That is very likely the association you missed