Project project = HpdApi.GetInstance().OpenProject("project path");
Variant variant = project.GetVariants().First();

// Open drawing (Nailboard in this example).
Drawing2D drawing = variant.GetNailboards().FirstOrDefault(n => n.Name == "nailboard name");
drawing.Open(false, false);

// Find the desired 2D occurrence.
Base2DOccurrence nailboardOcc = drawing.GetAllOccurrences<Base2DOccurrence>().FirstOrDefault(o => o.Name == "nailboard occurrence");

// ID of the original occurrence.
string origId = nailboardOcc.OriginalOccId;

// Get the ID of the source document from which the occurrence comes.
string docId = drawing.GetSources().FirstOrDefault(s => s.Id == nailboardOcc.ParentWireHarness.Id)?.DocumentId;

// Open the source document.
Designer designer = variant.GetWorkspaces().Union<Designer>(variant.GetWorkdesks()).FirstOrDefault(d => d.ID == docId);
designer.Open(false, false);

// Find the original occurrence.
Base3DOccurrence occ = designer.GetAllOccurrences<Base3DOccurrence>().FirstOrDefault(o => o.ID == origId);