How to search one of Nodes?

I want to search one of ActivityNodes in the GoView.
How to distinguish And access among ActivityNodes?

In general, you can iterate over all of the nodes in a document, and check each one until you find the one that meets your criteria.
foreach (GoObject obj in goView1.Document) {
ActivityNode node = obj as ActivityNode;
if (node != null && MyPredicate(node)) { … }
For the case where you are searching for an ActivityNode with a particular Text/Label string, and the labels are unique (or you don’t mind duplicates), you can just use GoDocument.FindNode.