It is useful to identify the version of FileMaker you are using. I can tell from your description that you are using a version older than FileMaker 11 where what you get when you double click a field like this has changed.
Go to Manage | Value lists. You can find and edit the values for your custom value list there.
Thank you for your prompt response. I have cruised through all the menus, and cannot find anything resembling a Manage command. I'm in FileMaker Pro 8. The wonder of it all is that I managed to get in and edit the previous copy, but now can't find the way.
I'm in. It's Define, under the File menu.
The menus have changed since FileMaker 8 was released. It helps us to help you if you identify the version you are using.