Well, shoot. My programming cable doesn't fit.
I realize now that I bought the cable for the 5V and 3V "plus," but I only got the 3V radio, not the "plus." Other than the different plug config, I'm not sure the difference between a plus and a non-plus.
So I just ordered another cable, the non-plus cable. In hindsight, I should have got the 3V-r plus, so I could have used the same cable for it and the 5.