I LER it is possible to add ValidValues to attributes/fields.
But this tab is not available in Domains.
You can substitute data type by domain in attribute of entity. That replaces data type. But valid values can be set only in attribute and only for explicit data type, not when used with a domain.
To sum up:
Domains do not have ValidValues tab when appropriate data type is used.
If (1) is OK, then in attribute editor no Valid Values tab is available if used with domain whose data type supports valid values.
if (1) is broken, attribute editor still could contain Valid Values tab, but only as read only when using domain with type that supports valid values.