I was able to complete my enrollment, by following a hunch suggested from a Delta twitter response.

The enrollment popup requests your address. Just copy your Address Line 1 from your Delta SkyMiles profile. This will generally be your street and number, without a city and state. Allow the software to learn your City and State from your Zip Code. This worked for me.

The problem is either the software doesn't want your explicit City and State, or the line with the street, number, city, and state is too long. Anyway, the enrollment worked for me, with only Address Line 1 and my Zip Code.