I saw the same problem you did. I could SET, but after closing the Network Dataset Properties dialog, the properties that were set went back to the default values the next time the dialog was opened (which means that the Data PropertySet was null).
Did you try what I mentioned?
Open the network dataset properties
Select the attribute in question and click Evaluators...
Click on the source for which you want to set the evaluator
Click on the evalutors properties button
SET your properties
Click OK to close the evaluator properties dialog
****** Click APPLY on the Attribute properties dialog, then OK ******
****** Click APPLY on the Network Dataset properties dialog, then OK ******
Close the Network Dataset properties dialog
If for some reason, you do not see the APPLY button become enabled, then your properties are not going to persist and the next time you try to get them, that property set will be null.