It may not see it because that particular UX for some reason isn't compatible with the iGO version you're running.

Do you have other UX's working ok?
If yes, then the issue is most likely with that single UX that doesn't show up for you.
It may be that the UX you're not seeing doesn't have the correct resolution data listed in it's code, so it doesn't appear in the settings.
Skins have a similar issue that if the correct resolution data is not present in the skin the program will not display it in the settings.

If no, then the extra settings function may need to be enabled in the sys.txt file in iGO's root folder on the device.
In the sys.txt file look for an entry under the interface header:

[interface]
extra_settings=1

If it's not already in the sys.txt file add it then save the sys.txt file.
If that was the problem, iGO should now display an option in settings named 'Extra Settings' and in that you should find the UX you're looking for.

There may be another reason that particular UX doesn't show up that I am unaware of.
Perhaps trying to run a UX designed for WindowsCE on an Android device, or vice versa may be one reason.
I'm guessing that UX's made for Nextgen aren't compatible with the older Primo versions etc. etc..