Sorry, there isn't enough information to determine exactly what issue you're running into but I do have a few debugging suggestions.
In you web service result handler, print out the results which are returned from the server using the trace(mx.utils.ObjectUtil.toString(myResultObject)) method. This method will dump the object to flashlog.txt.
You could also try {wsGetNames.getDetails.lastResult.lastName.toString()}
If this is working successfully in a list where the dataprovider is {wsGetNames.getDetails.lastResult} then you should be able to user {wsGetNames.getDetails.lastResult.LASTNAME} for the LABEL or TEXT parameter.