I am not sure why the error actually says "Argument not optional", but I can see that the code isn't quite right. The error actually does not have anything to do with the BrowseBranches method - it has to do with the Add method.
Dim client As New EasyDAClient
Dim collection As DANodeElementCollection
Set collection = client.BrowseBranches("localhost", "OPCLabs.KitServer.2", "SimulateEvents")
The Add method can add one element to the collection - but not one collection to the other, which is what your code attempts to do.