Hi again Traci,
The problem is the name of the file. Change the suffix from ‘json’ to ‘geojson’. After I did that and dragged the file into your document, I got the desired boundary data, ready to be joined with the district level data.
I believe that without the name change CODAP is attempting to treat the file as a different CODAP document rather than something to be imported. (Long ago, CODAP documents had the ‘json’ suffix which was later replaced with the ‘codap’ suffix.)
Hope that helps and you have a successful join!
Bill