The actual types of objects for the arguments are missing (in bold below) --Mitch
Adds an arc curve, created from a start point, a start direction, and an end
point, to the document.
rhinoscriptsyntax.AddArcPtTanPt (start, direction, end)
start Required. 3dPoint The starting point of the arc.
direction Required. 3dVector The arc direction at start.
end Required. 3dPoint The ending point of the arc.