If you are configuring an input like dropdown/checkbox, the fieldforValue defines which field value from the pupulatingSearch result will be used as token value and the fieldforLabel define which field value from the popultingSearch result will be used to display in the UI. E.g. showing label "All" in dropdown and using "*" as token value.
I tested with Splunk 5.0.5 and it doesn't work if the fieldforLabel column is not present in the result. What Splunk version you're using?
It is used to display Label names. Say for example Names in a dropdown.
<fieldset> <input type="dropdown" token="host" searchWhenChanged="true"> <label>Host:</label> <default>SomeValue</default> <populatingSearch fieldForValue="host" fieldForLabel="host">| metadata type=hosts index=os</populatingSearch> <choice value="*">all</choice> </input>
The fieldForLabel and fieldForValue should be in resultset of your search.
Hope this will help you http://answers.splunk.com/answers/151189/how-can-i-get-the-current-time-in-hhmmss-as-the-default-val...
What you tried was output field of your search?
It works for me.
There are many such examples here: