This might help a bit. If you create a restriction evaluator, and then set the Type to "Field", you can enter an expression (the little hand holding the paper on the right) to restrict the attribute you desire. The expression is this:
restricted = False
If [RoadClass] = "2" Then
restricted = True
End If
value = restricted
Where the bracketed is whatever your road class field is called, and "2" is whatever attribute you want restricted. The value goes in the separate value box below the VB code box. After you rebuild the network dataset, you should have an option to use restrictions when routing.
Hope this also helps.
eric