Only fields of type Textbox or Menu can be set to appear in search forms as an interval: to allow you to select a minimum value, a maximum value or a two values interval for this field.
In order to appear as an interval in search forms, a field must have "Is Numeric" checkbox selected. Having this checkbox selected will reveal a new option in "Search Type" option for this field: "Interval". Select this option and save changes for the field.
For a textbox field you can also customize the search values list if you want, by adding a list of values to "Search Values" field. If you want to simply show two textboxes where people can enter their own values, do not fill any values here.