It just so happens that I have another mac that happened to have an older version of AirPort Utility installed. I was able to click the + button without any issues and have it bring up the sheet for adding a new reservation. Obviously, there is a bug in the new version that needs to be fixed, but it would be great to know if there is a workaround of any sort.
FWIW, I tried to duplicate your issue using AirPort Utility 6.2, but it will not let me use a range of 10.0.0.1 to 200. The setting automatically reverts to the default of 10.0.0.2 to 200.
If you have a few spare minutes and want to experiment, try changing the range using the new version of AirPort Utility to 10.0.0.2 to 200 and Update to save the settings. Then, test to see if you can add a new reserved address.
Your main router will automatically be assigned 10.0.0.1 by default....so you do not need to specify the ".1" in the DHCP range.
If this works....then the older version of AirPort Utility has the bug....as it should not have allowed you to specify a range of 10.0.0.1 to 200. My guess would be that the new version of AirPort Utility accepted the settings from the old version....but then wouldn't let you make any further changes.
Bob, thank you for the quick reply. I don't believe I ever specified the range originally. I believe it may have been a default, but I could be wrong. In any event, the range is not editable (on this screen at least). When I click on the Help menu and type the word Range to see if I can get help on how/where to set the range, no topics are found. How do I specify the range?
Bring up the same screen as you posted above and then click on Network Options at the bottom of the page. That will bring up a screen that looks like this, so you can edit it, click Save and then click Update.
The screenshot that I posted shows Apple defaults. Your screen would look like 10.0.0.2 to 10.0.0.200 once you make the change.