This is what worked for me. I had a spreadsheet at
Note that 1qZ6Sgu2qcKy-Rbc-L_5YN-zJD2jErOw2BUhur3-yWlk is the ID for the sheet.
I did "publish to the web" and the dialog gave me this url:
Now ignore that URL. This is the <link> I put in the head of my Exhibit page to access JSON from that spreadsheet:
<link rel="exhibit/data" type="application/jsonp"
Notice that it uses the ID from the URL used to access the sheet directly.
More recently, for a different Exhibit, I found I had to further tinker with a data link URL such that:
Notice that /od6/ has been replaced by the number of the sheet: /1/; and, the gid has been tacked on to the end.
Google keeps tinkering with stuff, and is not always very transparent about it.