It's not hidden. I added a few things to make sure.
lua Code:
tkClock = tkUI.CreateText("tkClock", Guiroot) --CT_LABEL
tkClock:SetAnchor(TOPRIGHT,Guiroot,TOPRIGHT,-10,-5)
tkClock:SetText("loaded") --To see if tkClock not hidden
tkClock:SetHidden(false)
tkClock:SetHandler("OnUpdate", function()
tkClock:SetText(GetTimeString())
end)