I'm missing something basic then obviously.
I mark the checkbox, I see only "Updated IP Lock Setting". I uncheck the box, I again see only "Updated IP Lock Setting". There is no IP Address or comma separated list shown anywhere.
I checked this on my wallet and get the same. So I used Firebug to inspect the html elements and I see the IP text and current IP fields present but they are display=none when they need to be display=block to show up. Judging by the code the intention is that they only show when double encryption is enabled. I say this because the class name for those two elements is "control-group double-encryption-on".
So either it's a bug or not intended to be available but not documented as such. I don't want to change my wallet to double encryption to test.
But I did remove the "double-encryption-on" class name and they do show up then. Don't know if that works.
---- aside
I wonder if it might be possible to also support cidr networks, eg. 1.2.3.4/24 or maybe easier than that a more "non-techie" style eg. 1.2.3.4-56 for a range.