Jeff,
It sounds to me as if your U16 ring control is, or is linked to, a typdef (Type Definition), which LabVIEW programmers can employ to impose sharp constraints on the way a control looks or behaves. This would explain why you can't edit the text items, or add one. The bit about adding an item for '3' is confusing, but maybe it has to do with the ring being set to allow undefined values.
I can't recall how to manage these things under LabVIEW 5.1, but in 7.x, I would right-click on the control and see if "Open Type Def" was one of my options; if so, this diagnosis is correct, and you can make your changes only be editing the actual type def. The changes would then automatically be applied to any other control or indicator that is linked to the same type def, which is a very powerful programming technique.
Your best move might be to read up on Type Definitions in your LabVIEW User Manual, or in
Application Note 159, and then proceed.
Hope this helps,
John